External entities are any legal persons outside CERN.

In jurisprudence, a natural person is a person (in legal meaning, i.e., one who has its own legal personality) that is an individual human being, as opposed to a legal person, which may be a private (i.e., business entity or non-governmental organization) or public (i.e., government) organization.

Typically, legal persons can sue and be sued, own property, and enter into contracts.

Example: CERN's clubs and host states' authorities are external entities, the missions of CERN's member states, private companies and CERN's contractors as well.
However, CERN's members of the personnel and their family members, members of CERN's clubs are not legal but natural persons.