A simple overlap of sessions can cause an opportunity to not be billable. So users have to be very aware of that when entering session hours - a simple overlap of 1 minute can cause an opportunity to not be billable.
Example - the following scenario has a session overlap - one session ends at 8A and the other begins at the same time:
Example - the following scenario does not have a session overlap - the session end time and start time are not the same:
