An exclusion is a requirement of the standard that your organisation determines is not applicable, and formally omits.
What can be excluded
ISO 9001:2015 allows any requirement to be considered for non-applicability, provided the exclusion does not affect your ability or responsibility to deliver conforming product and services and enhance customer satisfaction.
In practice, clause 8.3 — design and development — is by far the most common. An organisation manufacturing entirely to customer drawings genuinely has no design responsibility.
What cannot
Anything you actually do. Excluding a clause because it is inconvenient, or because the system is not built for it yet, is a nonconformity rather than an exclusion — and an obvious one, since the auditor will see the activity happening.
Nor can you exclude a requirement to avoid a problem. If purchasing is difficult to control, clause 8.4 still applies.
It must be documented and justified
The exclusion, and the reasoning behind it, must appear in your documented information — usually alongside the scope statement. “Not applicable” alone is not a justification. “The organisation manufactures exclusively to customer-supplied designs and holds no design authority” is.
A cost consequence worth knowing
No design responsibility is one of the recognised factors that can reduce certification audit days. A properly justified 8.3 exclusion is not just tidy — it can lower what you pay for three years.