Am I understanding the implication of knowing a user ID of a file on the accessibility of that file on other filesystems correctly?

In [_Classic Shell Scripting_ from O'Reilly](https://learning.oreilly.com/library/view/classic-shell-scripting/0596005954/) , Arnold Robbins and Nelson H.F. Beebe write the follwing, > If a filesystem with user smith attached to user ID 100 were mounted on, or imported to, a filesystem with user ID 100 assigned to user jones, then jones would have full access to smith's files. This would be true even if another user named smith exists on the target system. and I'm not sure I really understand the implications of this, honestly. Does it mean that if I have a flashdrive, I mount it, and I cp on it a file which myself have created on my system, then that flashdrive can be umounted, mounted on antoher system with a user name myself (maybe created ad-hoc for the purpose), then that user has the access I had on my system to that file? --- Now that I wrote it, I start thinking there's nothing strange with it, in the sense that the file was not encrypted or anything, and myself cping a file on a flashdrive does mean that I'm trying to share it, so there's nothing wrong/unsafe in it becoming readable elsewhere. Am I missing something?