Release Access Rights can be found in the "PLC" and "Protection" menu command.

This is best explained with an example. If you have two programs in your project and both target the same PLC, and both are connected to that PLC, then you can download one of the programs. If you attempt to download the other program at the same time there will be an access rights conflict. The second download will be prevented and the on-line status of that program will "lock-up". The second program is not informed of the completion of the first program download, so the status will stay locked-up until the "Release Access Rights" has been selected.

The two downloads could of course be coming from different PCs or different devices.