The only way you could do that if you want to separate it by ony one user would be to create 50 usergroups and assign each user to a different usergroup and set permissions accordingly. Our application uses standard permission structures like most of the other software out there. It is all done by usergroup permissions and you assign users to specific usergroups.
Thanks Chuck. One additional question: is there a limit concerning usergroups? I would like to do it as you described and I hope that we will have hundreds of categories, maybe more..