The current representation of Reusable Subprocess / Call Activity in Stunner is like a regular collapsed subprocess.

According to BPMN2 spec, Reusable Subprocesses / Call Activity should have a thick borders that differentiate it to a regular subprocess:

We should change the shape of call activity to use thick border as stated by the spec: BPMN2 Page 184:
"The details of the called Process can be hidden and the shape of the Call Activity will be the same as a
collapsed Sub-Process, but the boundary of the shape MUST have a thick line"