ActivityDefinitions are used by the DSF to advertise which processes are available at any given instance and who is allowed to request and who is allowed to execute a process. The DSF defined elements for this purpose in the dsf-activity-definition profile.
The most important elements in ActivityDefinitions are:
message-nametask-profilerequesterrecipient
The message-name element contains the name of the BPMN message start event or
BPMN message intermediate catching event which expects
a Task resource complying to the profile defined by task-profile.
The requester and recipient elements define the organisation(s) or person(s) who are allowed to request or receive the message
specified by message-name. The receiving DSF instance is the one who will execute the process connected to the message.
You will have to create your own ActivityDefinitions when developing a process plugin.
If you are fluent in reading XML FHIR definitions and translating them into XML resources, you can take a look at the
DSF's profile for ActivityDefinitions here.
ActivityDefinitions also reference other resource definitions. Depending on the resource, you will find them in one of these folders.
If you are not as comfortable with these requirements you might want to check out the guide on creating ActivityDefinitions.
You can also find examples for all possible requester and recipient elements here.