Tasha’s Cauldron of Everything offers the following rule:
If a creature falls into the space of a second creature and neither of them is Tiny, the second creature must succeed on a DC 15 Dexterity saving throw or be impacted by the falling creature, and any damage resulting from the fall is divided evenly between them. The impacted creature is also knocked prone, unless it is two or more sizes larger than the falling creature.
However I am unclear how this would relate to resistance, the Monk’s Slow Fall, and other damage reducing features.
The rule says "any damage resulting from the fall is divided evenly between them" but it doesn’t specify whether that damage is split before or after damage reducing features.
Is the damage divided between the two creatures before or after damage reducing features are calculated?