Possession of a key for a higher service level typically includes {TAIL ~ keys} for {HEAD ~ lower levels}, or low service level keys can be derived from a higher service level key but not vice versa 
Possession of a key for a higher service level typically includes { TAIL ~ keys } for { HEAD ~ lower levels } , or low service level keys can be derived from a higher service level key but not vice versa
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH