Queue: Paul M. Weir in 3, 2, 1...
...but while we wait, I think a 'factor' in this is that PzIII and IV have the side hatches...
...also III and IVs have top hatches for the driver / co driver (though not 'spring' assisted best I understand)...
...and finally you are DFE on not the whole crew but the majority of the crew or maybe even half the crew...
...and now over to Paul....
It's a bit impressionistic, IE what the designer thought right.
There are a few patterns that I noticed:
(1) The CS# minimum is usually the number of crew, the T-35 with 10 crew only getting a CS8 being the most notable exception.
(2) Having at least 1 AF of 6 or more often gets you 1 extra, with some really heavily armoured like the IS series getting 2 extra (CS6 for 4 crew).
(3) A CS# of 6 is the usual limit.
Eg, going on the IS pattern the Tiger II should get a CS7 by (2) but gets CS6 by (3).
After that there is a lot of gut feel. The presence of armoured bins, how high is the ammo stored, petrol or diesel, number and placement of hatches might have pushed the designer to grant or deny an extra 1 to the CS#, but crew size and overall armour seems to dominate.
OK, Panzers as illustration:
Pz I to Pz 38(t), CS = Crew size. Nothing particularly good/bad about hatches compared to other most tanks.
Pz III: The Pz III E to very early III L had driver and BMG gunner hull side hatches but no hull top hatches. So Pz III A to G get CS5 with H & J getting CS6 due to thicker armour. Most L, N and all M had no hull side hatches. So the hull crew had to use the 3 turret hatches or the small transmission/brake access hatches in the hull front roof (in front of the driver's plate). So the CS goes back down to CS5.
Pz IV: Each crew member had his own hatch in all versions. So CS5 to Pz IV D and CS6 afterwards.
Panther to Tiger II: Hull crew had their own roof hatch. All had a turret rear emergency hatch but in the Tiger II that could be awkward to use. The commander had a hatch and in the Tigers the loader had a hatch. The gunner had none. However that was and is to this day the standard, 2 hatches for 3 turret crew.
Ammo armoured bins: With the
possible exception of the Pz II, as far as I know, the Pz I to Pz 38(t) had mainly unarmoured stowage. From at least '40 the Pz III and Pz IV used lightly armoured bins for most ammo stowage. The Panther and Tigers had unarmoured ammo stowed in the side extensions above the tracks and the Tiger II had ammo in the turret rear.
While having armoured bins, no ammo above the tracks, diesel engines or even hatch numbers could in real life make a significant difference, these don't seem to have made much difference in ASL except for some
Red CS# vs Black
CS# cases. Crew size and heaviest armour seems to be the dominant factor.