The Force user and group capability forces each authenticated user to function as the imposed user/group.
This function does not apply directly to file permissions, but it is related because a different user is authenticated and a different user accesses the files. For example, on a share, we have a document called Document_A with access only for user Roland and we choose Kent as the imposed/forced user. In this case, if Roland enters the share he will not have access to this file because Kent does not have the appropriate permissions. Also, if Roland creates new files, their owner will be Kent. The same will happen if Kent, Caroline or any other user enters the share - they will be working as Kent.
This is similar to the Superuser function, but in the case of superuser, all selected users are working on all shares as user root - to have full access to any file.
Please note also that Force user and group applies only to the SMB protocol. Beginning with DSS release 5.0 DB47, you can use Squash user and Squash group that will work similarly for the NFS protocol, provided all-squash or root-squash are also enabled in the share.