Modeling Processes, Sub-Processes, and Tasks in BPMN business process diagrams
An activity is work that an organization performs. There are three types of activities: Process, Sub-Process, and Task. You can model these activities on BPMN diagrams.
Process, Sub-Process, and Task are all graphically depicted using the same symbol. The use of different nouns reflects the hierarchical relationships between them. However, the Transaction process is represented differently.
•A Process is a network of “doing things”.
•A Sub-Process is a decomposition of a Process.
•A Task is a decomposition of a Process that itself does not have any further decomposition. In other words, the Task is the lowest-level process.