Hi all
After some tests it seems I didn’t give this thread the best title according to what I had in mind.
What I’d like to have the possibility is, from the point of view of the newcomer device to a pre-existent folder share, to get a list of files in the share and only get synched with the ones the user at keyboard on this newcomer device wants. To be more precise, Alice is the original files owner, Bob would get some of the files Alice offers… and last but not least, Alice is not Bob 
As long as both devices know there exist differences from the other about this folder share, maybe the remote one would be able to show this list in it’s own GUI or some unsynched file like .stignore is. When enabling a prexisting share on a device, we would be prompted to sync all/none/select/postpone. This seems to add a huge complexity to ST, but at the moment, the only way I found to do this is to call the newcomer with phone or mail, tell him I’ll propose (s)he to the share, tell she/him the name of an index file (eg _index_DevID) I already put in the folder (based on external operation, manual or cron or .job for updates), instruct him/she to create the target in her/his tree, and tell to create the .stignore as
!index
*
If I don’t do this, the sync starts for * as soon as the newcomer ends its new repository config.
I tried here to bring an answer to Nutomic’s suggestion in a share with people thread and on demand link there in:
This post is not android specific. One thing I love in ST is it is multiplatform. As may has yet said, you already did a very great work.