XPDL contain most of the entities, which are likely to be required in the process definition modelling. Sometimes, there is a need for some additional information (user or vendor specific). Extended Attributes are the primary method to support such extensions. They are attributes those defined by the user or vendor, where necessary, to express any additional entity characteristics.
Extended attributes can be used in all entities which supports it by XPDL. TWE provides use of the Extended Attributes like follows: you can define "simple" part of Extended Attribute by entering Name and Value attributes. If you want to define "complex" part of Extended Attribute, which could be consisted of tags that belong to the XPDL or some other namespace, you have to enter it as a free-text in the Complex content field.
Here is a picture of property panel representing extended attribute:
You can see at the picture above, when defining new ExtendedAttribute (or modifying one already defined), user have possibility to choose its name amongst the names of extended attributes already defined for that element type (Activity, Participant, ...). The list of names depends on the names already defined in opened XPDL (and its externally referenced XPDLs). The list of names is calculated in the memory each time new ExtendedAttribute is being added or when one already defined is being re-defined (the list is not persisted anywhere).