Apologies guys for adding to the replies, but at my company one of the main reasons for certain callsigns are due to call sign confusion avoidance. Our guys in flight planning analyse all other known flights to operate on the same sectors as the flight in question including those during climb and descent. Then they pick a callsign which tries to follow the guidelines mentioned in the AIC mentioned above. Resulting in a few wierd ones.
I have never seen any document to answer the question asked, sorry.
