A 'Sub Master Key' is used to open smaller groups of locks within a large master keying system. You can have several levels of "Sub Masters" within your master key system.

One example could be a larger office building that has a sub master for each floor that sub master would open all the locks on that floor and that.

A 'Change Key' is a key that usually operates one lock, one office or a group of locks that access the same area. For example, an employee could have a "Change Key" that would only operates their office. This key could be changed or "rekeyed" if this employee left your company without effecting the "Master Key" or "Sub Masters".

An employees entrance door could be keyed to a "Change Key" and passed out to employees. This key could also be changed or "rekeyed" if you have employees leave. Also without effecting the "Master Key" or "Sub Master" keys.

'Cross Keying' is a method that allows several keys in your master key system to operate a common area. One example could be to have a supply closet "Crossed" with several office keys, so several employees could access the supplies when needed with each of their own individual offices keys.