Hi Jean-Bernard ,
Thanks for your post.
You can read here the plugins we ensure ATUM is 100% compatible.
That doesn't mean that it couldn't work with other plugins. It means that we didn't test it, and we don't provide support on any incompatibility issue.
That said, I doubt a POS plugin can work with ATUM Multi-Inventory (unless explicitly stated) because we store secondary inventories data in custom tables.
FYI, we plan to release a mobile app in the next few months. It includes a POS module fully compatible with all our add-ons.
Best Regards,
José Andrés