Relation between FortiOS version on Fortigate and Forticlient package registered to it ?

I have googled myself silly and read through the Forticlient manuals but i cannot find any solid information on any relation between the Forticlient software version and the FortiOS version it is registered to.

Right now my users have Forticlient 5.4.0 installed, registered to a Fortigate cluster running FortiOS 5.2.3

(using old style forticlient 5.0 licenses installed before upgrading to FortiOS 5.2.3)

These licenses sadly become unusable when upgrading to the 5.4.x software train so the Fortigates will stay on the 5.2.x release train for now until we have time/resources available to look into the whole FortiEMS thing.

But what about the Forticlient software ?   i have yet to find any rule stating the relation between those 2 ?