A more thorough explanation:

Term: Forinsec Service
Definition: Forinsec service refers to feudal services owed by an intermediate lord, especially those of a military nature. It is also known as foreign service or forinsecum servitium. This type of service is different from intrinsec service, which is the service owed by a tenant to their immediate lord based on their agreement. Forinsec service is the service that is required for the tenement while it is in the lord's hand and is not part of the agreement between the tenant and their immediate lord. The term is relative, meaning what is intrinsec between two parties may be forinsec as regards a third party.
Examples: In medieval times, a lord might require forinsec service from their tenants to provide soldiers for the lord's army. For example, a tenant might be required to provide one knight for every tenancy they held. This service was not part of the agreement between the tenant and their immediate lord but was required by the intermediate lord. Another example of forinsec service is when a tenant is required to maintain a bridge or road that is not on their land but is necessary for the lord's use. These examples illustrate how forinsec service is a type of feudal obligation owed by a tenant to their intermediate lord.