NTuser.dat is the portion of the user profile where the registry settings for that user are stored. When it is named .dat, the user is able to make changes to the desktop environment (i.e., the user profile) and those changes will be saved when the user logs out.
NTuser.man is the same thing as NTuser.dat, but it has been manually renamed to .man in order to change the profile from user-configurable to a mandatory profile. Profiles with NTuser.man can be employed by multiple users simultaneously, since no user changes will be saved into the profile when they log off.
You, as an administrator, can change the name of NTuser.man to NTuser.dat and vice versa as needed.
This was first published in February 2003