This should be explained in the functional documentation. Have a look at the documentation for “Enter Repository Location”. You should have a look at it. I don’t want to copy it all in here, but here is a small text snippet from that documentation that is relevant to your question:

The status for a repository will be either 'Usable' or 'Generating'. Generating is the location where all new document files are stored. A given document class can have only one location in 'Generating' status; all other locations must be set to Usable. 

Perhaps that will answer your question or perhaps you wonder why we use that status for the move functionality. The answer to the latter is simply that we do not want new documents to be added to the “from” repository, once the process starts. I hope this makes sense.

Thank you @Mathias Dahl!