The left-hand menu of the Listing Media interface displays filters for each MLS connected to the listing and provides access to the Media Archive.
The "Active" filter simply shows all photos that have been uploaded for the listing, as long as they have not been archived.
Each MLS has different rules about how many photos may be published with a listing, so the filters display the current number of photos uploaded and the maximum allowed number of photos for each MLS. If the current number of photos exceeds the maximum allowed by that MLS, the filter label turns red and a warning appears along the top of the page. This indicates that you should remove some photos from publication to that system.
To select photos for a system, click on its filter. You can remove photos for publication to that system by selecting them (using the selection circle) and then clicking on the "Do Not Publish to" link that appears in the blue control bar at the bottom of the screen.
Photos that are not selected for publication to a particular system will be dimmed when that system's filter is selected. Also, the name of that system will not appear in the upper right-hand corner of the photo when you hover your mouse over the photo.
You can add photos back into publication by selecting them and then choosing the "Publish to" link that will appear if you select a set of photos that are not currently set for publication.