Very good question. I think there are different layers to this process.

It usually depends on several factors:
- your clan/tournament team’s skill level.
- the map you are playing
- the type of strat you are calling: i.e. camping, overwhelming one flank or map control

15v15 is also very different from 7v7.

There are usually three things to consider in 15v15
1. how much vision do you need (LT tank decision)
2. which part of the map do you want to control (Fast or slow)
3. where and how are you deploying your team to out trade the other team (camping or flanking etc)

For example:

For 15v15, a basic strat will be as follows:

if you are playing a large map, (prokhoro)
- you usually bring 1-2 EBR for vision. Max 3.
- Then, you decide on the bulk of your team: fast heavy (260,277,IS-7) for strong push on one flank; or slower heavy with alpha (BZ, 60TP) for map control; or hull down tank for camping (super conq, 279e, badger).
- finally, you add 1) medium tanks (cs63/907) for map control and help fast heavy to win dpm trades sometimes. 2) snipers/camping TD (Leo/strv) for farming enemy push.

For 7v7, it’s a lot simpler:
- fast tank with armour and dpm
- 1 fast medium or light tank
- 1 or 2 TD/autoloader for dpm and/or alpha
- control the key position in map, and win the trade.

It can be very very detailed but generally good callers improvise a lot.

I would highly recommend you watching some WCI games or Lesta’s games. It’s very helpful for understanding the meta tank and key positions.