Checking this box changes the way resyncing the POSM database works. This will only run the resync on sessions that have session modified = true. It was originally for server customers who needed to make changes in the database, and only resync the sessions that they modified. To save time and not have to resync the whole database.
These functions are broken down into buttons in session management to further provide control of the data sync: Sync Special Fields, Update Distances, Recalc Weight and Recalc Count Groups.