Activities are tasks that must be executed during the process. They can be performed by an individual or the system. An activity is depicted with a rectangle with rounded corners. Activities can have sub-processes, loops, multiple instances, and compensations.
A sub-process is a set of activities that can show a task in more detail. For example, the activity "deploy web page" may include the sub-tasks of checking it into a source control, merging code to the production servers, and copying from production to the live servers. A collapsed sub-process is depicted with a small plus sign on the activity symbol and the sub-process is visualized in a separate diagram.
A loop repeats until one or more conditions are met.
An activity with multiple instances means all subtasks must be completed before proceeding to the next task. These instances can be executed in sequence or in parallel. For example, you might model a process during which 3 stakeholders have to approve an outline before a video can move into production. Those three approvals would be an activity with multiple instances.
A compensation event appears in the process in response to something that needs undoing such as a cancellation or denied authorization.
A transaction activity is a special type of process used for payment.
A call activity is a reusable sub-process throughout the entire system.