It prevents having multiple different tools or items in the same location throughout the whole device (system). This would be useful as we only want one type of item stored in each location and not multiple as it requires you to check the other items and sometimes we have accidentally booked 2 different items to the same location causing confusion.
We will add this. In general we have the capability to assign multiple items to same location so that we can optimize the use of locations. To minimize the risk to assign multiple items to same location the "Show occupied locations during assignment" on device settings can be unticked.
By doing that the location list will as default always show only empty locations during assigning.