refer the implementation guide, page 200
Computer mode:
The client computer gets the policies from the group of which the computer is a member. The client protects the computer with the same
policies, regardless of which user is logged on to the computer. The policy follows the group that the computer is in. Computer mode is
the default setting. Many organizations configure a majority of clients in computer mode. Based on your network environment, you might
want to configure a few clients with special requirements as users.
User mode:
The client computer gets the policies from the group of which the user is a member. The policies change, depending on which user is logged
on to the client. The policy follows the user.