An occurrence is in its widest sense anything that happens. This includes human activities, but also physical and (bio)chemical processes, control processes, events and physical phenomena. Processes includes both discrete processes and continuous processes. A discrete process consists of a sequence of discrete process steps or actions. Examples of discrete processes are business processes in which various persons provide a contribution in the process, mechanical production processes in the mechanical industry and batch processes in the chemical industry. A continuous process is a process that is an operation on a continuous stream of raw material to produce one or more continuous streams of product materials. Examples of continuous processes are the flowing of water in a river, the operation of an electric power generation plant and the various refinery processes. An occurrence in fact always happens over time, but in some cases, such as events, the duration can be neglected. All these kinds of occurrences can be modeled in Gellish in a similar way.

Occurrences, and its subtypes: activities, processes and events, can be classified by kinds of occurrences, whereas the names of those kinds consists either of nouns or of verbs. For example, a particular individual human activity can be classified as a 'walk' or as 'walking'. Both kinds of terms are considered as being equivalent denotations of the same kind of occurrence. Therefore both such terms are included in the Gellish Dictionary as synonyms. Kinds of occurrences can be denoted by terms in an active form, a passive form and a noun form and sometimes other forms. For example: act, acting and action, but also activity. All such forms are treated as synonyms in Gellish.
Each individual activity, process or event shall be classified explicitly, for example as follows:

An occurrence, activity, process or event is further specified by describing the kinds of behavior of the objects that are involved in the occurrence, as well as their mutual interaction. This means that an occurrence is an interaction between two or more objects. Thus more than two objects can be related because of their participation in one occurrence. It is even possible that a variable number of objects are involved over time in one occurrence. For example in a project or a meeting. The modeling of occurrences therefore requires that it should be possible that more than two objects are involved in one occurrence. This can be achieved in Gellish by modeling occurrences as 'objects' that represent higher order (or variable order) relations, being relations with typically more than two related objects. The details of an occurrence are expressed by the specification of the objects that are involved in the interaction, especially in the various roles that they play.

An occurrence is modeled as a higher order relation, because an occurrence is an interaction of several objects. This means that several objects are involved in an occurrence, each with its own role, although often only the involvement of one or two objects with their roles are specified. The roles that various objects play in an occurrence are modeled in Gellish by the specification of a different expressions for each involved object. Each of such an expression specifies that an object plays a particular role in the occurrence. This results in several lines with involvement relations. The involvement relations are described by expressions that use kinds of relations that are subtypes of the <is involved in> relation type.
For example:

The Gellish English Dictionary contains a large number of kinds of involvement relations that describe ways in which objects can be involved in occurrences, processes and activities. For example:

• is performer of
• is enabler of
• is input in
• is output of
• is subject in
• is tool in
• is enabler of
• is controlled by
• etc.

If only the two roles are of interest, the roles of a performer and of a subject, then it is allowed in Gellish to express the occurrence as a binary relation. For example, the idea: 'pump P-101 pumps fluid S-1' can be expressed as follows:

The UID of the idea (in the example idea 101) is the identifier of the occurrence. In this example the individual occurrence is not given a name. However, as a guideline it is recommended that software generates a name that is the concatenation of the name of the kind of relation, the string ' of ' and the name of the individual subject. In the example the name of the process would be ‘pumping of S-1’.

Occurrences that do not exist yet in the Gellish dictionary can be added in a similar way as defining physical objects by obeying the [wiki:“Proper definition of a concept” rules for proper definitions]. A kind of occurrence shall be defined as a subtype of an existing kind of occurrence and given a textual description, whereas facts that are by definition the case shall be added to complete the definition.
Note that there is a distinction between generic kinds of occurrences and kinds of occurrences that have by definition an intrinsic performer or enabler. In general, generic kinds of activities and processes are independent of the subject of the occurrence, and independent of the performer or the enabler. For example, the activity 'design' or 'operate' does not include an indication of what is designed or operated. However, more specialized ‘qualified’ activities often do include the subject of the activity or process. For example, “design a pump” or “operate a facility”. In such a case the definition of such a kind of occurrence should include and explicit relation that states that it has by definition as subject a particular kind of thing. For example the concept 'design a pump' is defined as follows:

Similar considerations apply for individual activities that include a specific individual object as its subject. For example, the activity called 'design of P-101“ is by definition a design of a thing called P-101. This is specified as follows:

Note that the kind of relation 'has by definition as subject a' classifies a relation between two kinds, whereas the kind of relation 'has as subject' classifies a relation between two individual things.

Often it is required to specify in which sequence activities or chains of activities shall be or are performed. This may result in activity networks. For example, a sequence of activities that specifies a schedule for testing, commissioning and start up of a facility or the activity sequence that describes a maintenance procedure. Sequences can be specified for kinds of activities, but also sequences of individual activities, or in procedures in which a typical (individual) activity sequence is described that shall be repeated every time that the procedure is followed.
For example, a generic activity sequence that describes steps that shall be taken during a planned maintenance shut-down (also called a turnaround) of facility may specify a sequence of kinds of activities. For example as follows:

Typical activities are activities about individual things. Usually they are modeled as individual activities. When used in a procedure, the represent imaginary activities, whereas the real activities repeatedly use the procedure as a template. A sequence of typical individual activities uses other kinds of relations than above. Such a detailed procedure is expressed as follows:

Further details, for example about specifying partly overlapping activities, are given in the book 'Semantic Information Modeling Methodology'.

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