1 session is a minimum of 30 minutes of work during an update. Clients are encouraged to group update requests together to fully take advantage of the allotted time in the plan.
For example, if you needed 3 items updated on your site and you sent the request all together in one email, assuming it took less than 30 minutes to complete all 3 items, this would count only as 1 update session. Conversely, if those 3 updates were sent individually, then that would be counted as 3 update sessions.