H2020: Should the amount of institutional costs be reported according to the flat rates provided or correspond to specific invoices?

Institutional unit costs are calculated according to  the researcher unit costs and should be reported as amount per unit x number of implemented person-months (declared for the researcher cost).

In the Annotated Model Grant Agreement one can find the following explanation:
In practice the declaration of institutional costs for MSC grants is completely automatised because the IT system automatically calculates them on the basis of the number of person-months declared for the researcher cost.” (p. 380)