A seam is a place where behaviour can be changed or redirected without dismantling the whole system. Useful seams may appear around a business capability, an integration, a data flow or a user journey.
Look for business boundaries
Technical layers rarely make good modernization units. “Replace the data layer” crosses too much behaviour at once. “Move customer notifications” is narrower, observable and tied to an outcome people recognise.
Modernize one meaningful capability end to end, not one technical layer everywhere.
Control the transition
Introduce a stable interface, route a limited slice of work through the new path and compare outcomes. Preserve a fallback until the replacement has earned trust. The old and new systems will coexist, so design that coexistence deliberately.
Choose the next seam from evidence
Each extraction teaches the team about coupling, data ownership, testing and operations. Use that evidence to select the next move. A modernization architecture should become more specific as the work proceeds, not pretend to know everything upfront.