Hello Fred,
This image is a backup what i have from the old forum.
For your case follow the flow of example.
Create a empty USER B Without Access Control List(ACL), Create a group B without ACL, put the user B in the group B, create a Role B without permissions, link the role B with the group B, Create doctype B and add a ACL for the doctype B with the role B with permissions create doc, create doc files, view doc, view doc files… and repeat … create source B or use the default source, add ACL to source B(or default) linked with role B, and add to ACL permissions create documents, create new document files.
Sorry for my bad english.
Greetings.