Built in rule conditions in DTM that are built around session count use the session cookie _sdsat_session_count. This cookie functions like most browser session cookies. If the user is idle for a significant amount of time, but hasn't closed the browser, then it's treated as a single session. It is important when comparing session count to other unrelated tools as they can be incrementing sessions in another way. For example, another tool could be using a session timeout, thus session count could be much higher.
Related DTM rule conditions: