At IMCF, and ACS, the rules are very similar to HMBIA but with some distinct changes. Below you will find the rules from their official pages. HACSA did not create nor does it endorse any specific ruleset over another.
ACS Ruleset can be found here https://armoredcombatsports.com/
IMCF Ruleset can be found here https://www.medieval-combat.net/rules