The key is the identifier for the specific Branch.  It is used to form the Name, which in turn is used for form the Segment and then the Identity.

It can be from 1 to 63 bytes in length.  The data is stored as UTF8 and therefore may be limited to less than 63 characters when using values beyond ASCII.  For example Шофар (Russian for Shofar) would take 10 bytes and not 5.