As locksmiths, we use the terms such as Sub-Master Key, Change Key and Operating Key, not because we chose those terms, we were taught what they meant.
For our customers, these terms and others are usually confusing or have the wrong meaning to their understanding.
Instead of using these terms, why not use language the customer understands such as a Unique Key for a Change or Operating Key. The Unique Key is unique, operating just one specific bitting. It does not change and every properly cut key operates at least one lock.
Why not use the term Master Key, not a Sub-Master Key just because the term was used by the person who taught you. Even if the key is a low level Master Key, it is still a Master Key. This eliminates confusion and does not make the customer wonder what you mean by “sub”.
The bottom line is a customer and their employees including the maintenance personnel, who feels they understand the names of their key system, is a happier customer.
Your thoughts are appreciated.