External experts are usually justified as other goods, works and services in Horizon Europe. However, this can depend on the type of contract your entity makes with the supplier. Usually this goes under other goods. works and services (C3) as this is a purchase of a service for a defined cost. But other entities can declare this cost under personnel (A2) if case it is a consultant. However, the contract with the supplier has to fulfill precise conditions: work under conditions similar to those of an employee in particular regarding the way the work is organised, the tasks that are performed and the premises where they are performed, and the result of the work belongs to the beneficiary.
GMGA Article 6.2 – A Personnel costs
A.2 and A.3 Costs for natural persons working under a direct contract other than an employment contract and costs for seconded persons by a third party against payment are also eligible as personnel costs, if they are assigned to the action, fulfil the general eligibility conditions and (a) work under conditions similar to those of an employee (in particular regarding the way the work is organised, the tasks that are performed and the premises where they are performed) and (b) the result of the work belongs to the beneficiary (unless agreed otherwise).
GMGA Article 6.2 – C.3 Other goods, works and services
Purchases of other goods, works and services must be calculated on the basis of the costs actually incurred. Such goods, works and services include, for instance, consumables and supplies, promotion, dissemination, protection of results, translations, publications, certificates and financial guarantees, if required under the Agreement.