A claim in respect of a physical entity (product, apparatus) may seek to define the invention by reference to features relating to another entity that is not part of the claimed first entity but that is related to it through use. An example of such a claim is "a cylinder head for an engine", where the former is defined by features of its location in the latter.
Since the first entity (the cylinder head) can often be produced and marketed independently of the other entity (the engine), the applicant is normally entitled to independent protection of the first entity per se. Therefore, in first instance, such a claim is always interpreted as not including the other entity or its features: these limit the subject-matter of the claim only in so far as the first entity's features are suitable to be used with the second entity's features. In the above example, the cylinder head must be suitable to be mounted in the engine described in the claim, but the features of the engine do not limit the subject-matter of the claim per se.
Only if the claim is directed without any doubt to a combination of the first and second entities, the features of the other entity are limiting for the subject-matter of the claim. In the above example, the claim should be written as an "engine with a cylinder head" or an "engine comprising a cylinder head" for the features of the engine to be considered as limiting the subject-matter of the claim.
For the assessment of claims directed to computer-implemented inventions, where a claim to a computer program refers to a computer (a separate entity), see F‑IV, 3.9.