What is the difference between an “incurred cost” and a “committed cost”? What is the implication for the partner?

Committed costs are those costs for which a legal obligation to pay exists (e.g. a purchase order / a signed contract etc).

A cost is incurred when the related services and works are performed, or, in the case of supplies, the supplies have been used for the purposes of the Action.
Article 8 of the General Conditions requires, among the eligibility conditions, that the costs are “incurred during the eligibility period”. This means that only the costs that are related to activities performed or to supplies used during the eligibility period are eligible. According to this principle the date on which the related costs were committed is irrelevant.