Here is the answer clients give when you ask what to migrate. "Bring everything." And in HCM that is worse than in Financials, because "everything" includes decades of employment history for people who left years ago. Not transactions. People — with addresses, identifiers, medical certificates and disciplinary records attached to them.
Three categories, and every candidate goes into one of them with the placement defended.
Must migrate: current workers, their current assignments, the structures they depend on, service and seniority dates, and anything a live entitlement calculation needs — that last clause is doing real work, and it is why Module 12 asked you to gather the entitlement rules.
Should consider: recent employment history for current workers, current-year absence balances, document records with live retention obligations, and terminated workers within a defined recency window — defined meaning somebody decides on a number and writes it down.
Usually leave behind: full historical employment records for long-departed workers, historical document records past retention, superseded structures.
The history question is the one that consumes the budget. How much assignment history do you migrate for a current worker? Full history is expensive and often unnecessary — hire date plus current state is usually enough, plus whatever entitlement calculations require. Ask what calculation needs it before agreeing to load it. Not "do you want it", because everybody wants it. If there is an answer, load it; if the answer is "it might be useful", you have found something for the leave-behind column.
And now the argument that actually wins this discussion, which most consultants never make. Migrating personal data you have no lawful basis to keep is a data protection problem, not just a cost. It is genuine, it is often missed, and it reframes the conversation entirely — "it will cost more" invites a budget discussion, while "we would be carrying personal data we cannot justify holding" invites the client's own legal adviser into the room, on your side.
Three more, each expanded later. Cleanse before migration, not after — duplicate persons, failed identifiers, invalid addresses, fixed in the legacy system or the staging layer and never in the target. The dry run is non-negotiable: full volume, into a test environment, before UAT, not a sample. And reconciliation is the deliverable — headcount by legal employer, by country, by department, signed off by HR, not by IT.