Hi

What is the difference between Primary & Secondary groups of users regarding file access permissions. ???

Regards

Your primary group is the group that gets used when you create files/directories. Any file/directory with a group id that you are a member of (primary or otherwise) you will have access to according to the group permissions.

You can modify the behaviour by setting the group id bit on directories, but that's the simple overview

Your Primary Group is the default group your account is associated when you login, this is the group with the Group ID that is the 4th field in your account /etc/passwd file. Directories and files you create have this Group ID.
Your Seconday Group is any Group(s) you are a member of other than your Primary Group.
All Group(s) are found in the /etc/group file and are created with the 'groupadd' command or any text editor, such as vi/vim.
Users are added to a Group with the 'usermod -G groupname uname' command. You can see the members of a Group by viewing the /etc/group file.
Users can change the current Group they are in with the command 'newgrp' and can check their current group with the 'id' command.
Access to directory/files is controlled by the permission bits of the directory/file and the Group that "owns" the directory/file.