XES does not prescribe a fixed set of mandatory attributes for each element (log, trace, and event); an event can have any number of attributes. However, to provide semantics for such attributes, the log refers to so-called extensions. An extension gives semantics to particular attributes. For example, the Time extension defines a
timestamp attribute of type xs:dateTime. This corresponds to the # time(e) attribute used in Sect. 4.2. The Organizational extension defines a resource attribute of type xs:string. This corresponds to the # resource(e) attribute used in Sect. 4.2. Users can define their own extensions. For example, it is possible to develop domain specific
or even organization specific extensions. Figure 4.7 shows that a log declares the set of extensions to be used. Each extension may define attributes that are considered to be standard when the extension is used.