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This is another organizational approach, when the difference between regiments, brigades, and divisions are measured in the amount of battalions. Which means the battalion is the organizational basis.
Also NATO symbols and the logic behind them - how they combine the simpler sings into a combination of sign systems to mark modern unit compositions - also worth an investigation.
This organizational thing is very flexible. Liek a brigade can be two regiments, and a division three regiments, but then a division can consist of two brigades. It highly depends on which country and when.