Originally Posted by From the samba users manual
Many of the strings that are settable in the config file can take substitutions. For example the option “path = /tmp/%u” is interpreted as “path = /tmp/john” if the user connected with the username john.
These substitutions are mostly noted in the descriptions below, but there are some general substitutions which apply whenever they might be relevant. These are:
session username (the username that the client wanted, not necessarily the same as the one they got).
username of the current service, if any.
Based on that, I believe the correct would be the session username, the Upper case U.
The ", if any." from the lower case u explanation, make me think that there are situations where the username is null or whatever.
I have %U (upper case) in all mappings here, just in case.