The material that matters most, because its failure, loss, or misuse would cause significant harm: to people, services, rights, money, or reputation. Often regulatory decisions, entitlements, contracts, serious incidents, and ministerial advice. Warrants stronger controls.
High risk/high value material is what would cause significant harm if it failed, was lost, or was misused: to people, services, rights, money, or reputation. Typical examples are regulatory decisions, entitlements, contracts and procurement, serious incidents, and ministerial advice; these areas warrant stronger controls and better metadata.
IAR turns terms like this into working practice: assets classified against your disposal authority, stewarded by position, and ready to evidence.