Ok I think I figured this out. The mission: "morning at the zoo" I have 3X TRPs to place.
1) The TRPs are only effective if I have a spotter to call fires on it.
2) The only way to call fires on it is using the "SPOTTER" (FO) unit all other unit leaders can't request fires on the TRP.
3) The mortar units can can target the TRPs, BUT only if they have a good LOS to the target location. other then that its blind fires.
4) TRPs give the fire a quicker responsive fire and concentrated fire on the location.
So hopefully I got this correct. If not please explain to me other ways to utilize the TRPs.