Here is the list of Division-sized units that may be removed when playing a Corps-sized Units Reinforcement Card (see 220.127.116.11):
• Nationalists: MIL, POL, CAV, INF and AFR units.
• Republicans: Non-Regional MIL (except POUM, which may
not be removed), POL (though not from the Northern Front)
and RPA units.
• Other types of Division-sized Units may not be removed to
fulfi ll this rule.
Would that also include the option to remove reduced, and/or regional units?
Reduced units may be eliminated (the Rulebook does not specify it, so I clarify this point). Regional units may NOT be eliminated, as indicated in the list above.