Sources can be locked to prevent modifying them or listing their contents without a PIN. In order to setup a PIN code, go to the source in question and click on ‘Setup Lock PIN’.
When a source is locked, it has to be unlocked first using the PIN code in order to modify it or add folders/albums to it. Once the update is done, the source can be locked again using the ‘Lock Source’ button which automatically happens when the app exits.
PIN code can be changed or removed when the source is unlocked by clicking on ‘Lock & Lock Options’