Here's my experience (not sure whether it is actual 'rules'):

Both AF and KL pre-assign seats >90 days out, long haul only.

I.e. for telephone bookings you can ask, for existing bookings you can call and get a pre-assigned seat. In the past they also were able to pre-assign at the FB desk in the KL lounges.

Emergency exits and Comf-Y cannot be pre-assigned.