4 minutes ago, Kubernes said:Not just the army faction, but the unit's faction too, from how I'm reading that part of the rules. If that unit doesn't, then neutrals only.
It mentions matching the faction of the army and the type of the unit. Unit type is going to be either Infantry, Cavalry, or Siege. It says nothing about matching the faction of the unit.
RRG, pg. 3:
'Upgrade cards are equipped to units in an army. However, some
upgrade cards have a unit type or faction icon, denoting that these
cards can only be equipped to a unit of the matching type in an
army of the matching faction.'
RRG, pg. 20:
'87.1 Each unit has a unit type icon in the lower-left of its unit card. This icon indicates the unit’s type and is used to determine what type of trays to use. There are three unit types: infantry, cavalry, and seige.'