Robin,
In BCS unit HQs must be activated (there is a chance for a second activation) in order for its sub units to move. BCS uses a simplified but very effective supply system. Units must be within a certain range of an established 'divisional train', known as a stream. This train must then trace its path back to a supply source. If a supply train needs to move to stay in range it becomes a ghost train for a turn, which impacts the ability to activate (especially if the unit is not 'Fresh' or has low fatigue. If two divisional trains 'cross the streams' in the forward areas (use the same road), it can affect activation. Formations build up fatigue through supply issues (crossing the stream and ghosting a train), by being mixed together, by moving, fighting, all of which affect activation. If formations get too jumbled up it can cause chaos (supply) and thus bog an attacker down, even bring an attack sector to a stand still, thus the need to keep individual formations moving on their own axis of advance.
Eventually, formations will build up so much fatigue that they cannot properly activate and they will need to rest in order to bring down fatigue. Even while functional, a unit with higher levels of fatigue may only obtain a 'partial activation' which limits how far a formation's units can move and the effectiveness of its attacks.
BCS does not 'count factors' and then apply the results in odds tables. Instead, attacking and defending units compare combat effectiveness with terrain, type of attack, supporting units and then apply the +/- mods to a table where you hope to destroy strength points and/or force retreats.
BCS is not perfect but the latest rules updates make it a pretty slick game. Its preferable to SCS, which is too basic where supply is concerned. OCS has more accurate supply rules but because both it and SCS use odds ratios for combat results its leads to "factor counting".
Here is a screen shot from Last Blitzkrieg showing the German attack in the south of the Ardennes from Clervaux to Diekirch. You can see how formations are not all mixed up looking to create the perfect attack.
If anyone is interested, I have BbF and would be happy to run through the system with you.
And yes,... the games are on VASSAL.
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