Encumbrance is how physically taxing activity is for a unit. It doesn't apply to walking or running in battle; it only applies to Melee Attacks, Flying, and Spellcasting. After a unit flies, or finishes a melee attack, the Encumbrance value on their stat sheet is added to their total accrued Fatigue. This value is also known as Melee Encumbrance.

Every unit has a Base Encumbrance, which is then added to by the Encumbrance value of their Armor and Shields to determine their Melee Encumbrance; if they're Mounted, however, the Encumbrance added by their Armor and Shields is halved (and then rounded down). This only applies to Melee Encumbrance. The mount itself then shoulders some Encumbrance while ridden.

Spellcasting Encumbrance is a different value, usually. This is the base Fatigue accrued from casting a spell, fatigue which cannot be diminished by overcasting the spell with higher levels of Magic. Spellcasting Encumbrance takes the Base Encumbrance, and then adds twice their Armor and Shield Encumbrance, with no discount from riding something.

If a unit somehow has zero Base Encumbrance, then their Melee Encumbrance will always be zero, but their Spellcasting Encumbrance will still change with equipment.

"Severe Heat" and "Severe Cold" both add to Encumbrance, and are treated differently from the Encumbrace from equipment; they're neither halved for Mounted Melee Encumbrance, nor doubled for Spellcasting Encumbrance. In this case, the second point of Heat or Cold adds +0.5 (rounded away in most cases), while the third point adds +1.5 (for a total of +2 in most cases). This happens regardless of what the unit's nation prefers. Resistances to Fire or Cold adjust a unit's tolerance to these scales from the bottom up; for example, 5 points of Fire Resistance would negate the effects of that second point of Heat, but not the third, while a unit with 10 points would negate the effects of both.

Most humans have a Base Encumbrance of 3. Mortals of extreme energy or endurance might have 2, however, while cold-blooded beings tend to have 4 at base. Demons typically have 1 Base Encumbrance, while Undead and Inanimates typically have zero.