Integrated systems exchange data and disagree. A unified system has one record.
Integration synchronises separate systems; unification means there is only one record, and only the second removes reconciliation work.
The benefit shows up in the awkward transactions — cross-channel returns, exchanges, and the customer who exists in one system and not the other.
Inventory across several locations is where unified commerce gets genuinely complicated, and it needs deliberate configuration.
It is only worth the switching cost if you have real cross-channel activity; a purely offline shop gets very little from it.
Running every sales channel from one system with one catalogue, one customer record and one inventory pool, rather than several systems exchanging data.
The full guide has the formats, the deadlines and the worked examples.