The Omada "Lock to AP" should have three options (the first two exist now on/off; the third should be added):

1. don't lock (off)
2. permanent / hard lock (clients will be denied when AP not connected) # current "on"
3. preferred / soft lock (clients will be allowed to connect to other APs when the locked AP is not connected)

The problem this addresses is when you want clients to only connect to a given AP (e.g. because you know they will have a better connection _most_ of the time), but you also want them to be able to connect if the locked AP is offline. i.e. to be allowed to at least try and connect to the wifi network.