beg to differs..ANGKASA xxx is for "Operational Tasking/ Mission" assigned to any transport aircrft, be it helicopter / fixed wng. They'll use ANGKASA for RT within M'sia but normally if crossing intrnational boundaries, RMF...romeo mike foxtrot .
For VVIP/VIP it'll become DIRAJA XX / PERDANA xx or so on depends who is onboard.
For the MEGA, AMAN, GADING, RIMAU, SINTAR, BADAK, CEKAL,PUTRA and whatsoever...it is solely to differentiate that it is a "TRAINING FLIGHT" callsign assigned to each squadron for pilots to carry out their Monthly Continuity Training, Base CX, Route Training etc.... sort like a 'Non Revenue' flight if referring to civil world.
.....nothing secret, we're sharing the same sky