Activity type: Subprocess
See: Business Process Model and Notation (BPMN), version 2.0.
BPMN 2.0 knows four types of subprocesses:

Subprocess (standard) (A ([standard] subprocess corresponds to the embedded subprocess in BPMN 1.x.)

Event subprocess

Transaction, and

Ad hoc subprocess
Each type of a subprocess can be displayed as

Subprocess (collapsed) or

Subprocess (expanded)
Collapsed subprocesses have a special marker displayed at the bottom of the corresponding subprocess symbol: