Occupation is an integral part of warfare.
With few exceptions, the only way to force an enemy to capitulate in a war is to occupy provinces. In the base game and in A House Divided this was rather simple, but in Heart of Darkness different armies can besiege the same province at vastly different speeds.
Occupation speed scales directly with how many brigades are in the occupation force, up to a maximum of thirteen brigades: a stack of ten units of infantry will occupy a province twice as quickly as a stack of five, but a stack of thirty will occupy a province at the same speed as a stack of thirteen.
While laying siege to a province, an invading army receives 80% of the Supply Limit that the province owner's armies receive. An army will suffer a base attrition rate of 2% while occupying a province; except in the cases of invasion forces too large for optimum efficiency, this is the only attrition they will suffer.
If an army includes a brigade with a Reconnaissance score, then it will occupy provinces more quickly. The bonus to speed is equal to 50% times the Reconnaissance value of the highest-Reconnaissance brigade in the army. This bonus is reduced by having less than 100% Reconnaissance Efficiency.
Cavalry, Dragoons, Hussars, and Planes (but not Cuirassiers, despite their being mounted soldiers, until an invention unlocked in 1919) provide Reconnaissance to their army. So long as at least one tenth of the army is composed of Reconnaissance units, then the army's Reconnaissance Efficiency will be at 100% and adding in extra Reconnaissance units will not speed up occupation.
If less than one tenth of the stack has Reconnaissance, then the army's Reconnaissance Efficiency will be equal to the percentage of the required brigades present (i.e. one brigade out of twenty would give the army a Reconnaissance Efficiency of 50%) and the increase to occupation speed will be lower.
A fort makes it take about 75% longer to occupy a province per fort level (i.e. a level three fort will add 225% to the required occupation length).
If an army includes a brigade with a Siege score, then it will reduce the effective level of a province's fort by one for every point of the highest-Siege brigade's Siege score. This effect is weakened by having less than 100% Siege Efficiency.
Engineers and Tanks provide Siege capability for their army. So long as at least one tenth of the army is composed of Siege units, then the army's Siege Efficiency will be at 100% and adding in extra Siege units will not make the army any more effective against forts.
An army's Siege value does not increase the speed of occupation in provinces that do not have a fortress.
In the base game and in A House Divided there was little use to using large stacks to occupy land with: supply limits were low, and the occupation did not speed up appreciably compared to a smaller stack. The Heart of Darkness expansion changed this dynamic significantly.