The following extract from the Network Installtion Guide may help explain.

Adding Polygon Layers
When a new polygon layer is added by a user, the following actions occur:
1)    A record is added to Map_Sheet to track the polygon layer.
2)    A new polygon layer is created in MapServer by copying the SheetSource.* files from the
Base Maps folder into the Object Sheet folder, renaming them then attaching them to the MapServer dataset. This method is used rather than asking MapServer to create the layer files itself because of a bug in MapServer resulting in occasional invalid layer files being created.
Note because the Map_Sheet table is shared between users, as is the Object Sheet folder, polygon layers are shared between users. In fact, when 2 users have the same base map open and one draws a new polygon, Recorder periodically checks for new polygons on the other client machine and they will automatically appear soon after they are drawn.