This is more appropriately answered in an Architectural forum. But I'll give it a shot: To allow maintenance, cleaning crews and the management access, there must be a system for them to open the door locks. So instead of carrying around a huge numbers of keys, all the cylinders can be opened with a single key. This also requires all cylinders to be the same model and manufacturer. The architect needs to specify this to the owners requirements.
And you can have master keys that open some of the locks and grand master keys that open all of the locks.
Or are you talking about the drafting keying system where you have a bunch of specific notes which are "key" numbered to relate to their specific details located throughout the drawings?
