Retiring a system is a records event: what must be kept has to be safely migrated or exported, and the process documented. Decommissioning without record export can cause loss of access and unauthorised disposal.
Retiring a system is a records event, not just an IT one: whatever must be kept has to be safely migrated or exported and the process documented. Decommissioning without record export can cause loss of access and amount to unauthorised disposal, so involve IM/RM early.
IAR turns terms like this into working practice: assets classified against your disposal authority, stewarded by position, and ready to evidence.