Door groups are exactly what they sound like - a flexible way to create groups of doors. The main purpose of door groups is to make it easy to create privileges / access rights for a whole group of doors, without having to remember and include all the individual doors. For organizations with a large number of doors, creating well thought through door groups should make it much easier to assign correct access rights to users.
A door group can express any logical relation between the doors in the group, e.g. site, floor, type of room, security level, geographical area or something else. A door can be a member in many groups at the same time, e.g. site, type of room, and security level.