[REQ] Reasons why Dynamic Data Source can only be set once per session

[pixi]

Dynamic Data Source are a really great way to dynamically handle other files. 
What I still have issues to wrap my head around is why the $$ can only be used once per session. If the $$ should point somewhere else the file has to be closed and re-opened again.

Anyone an idea what the technical background here is? I'd like to understand.

 

FYI: https://support.claris.com/s/article/filemaker-16---dynamic-data-source?language=en_US