What the W3C says (not required)
What you need to provide a conformance claim
Partial Compliance Claims
It is worth noting that Partial Conformance Claims [https://www.w3.org/TR/UNDERSTANDING-WCAG20/conformance.html#uc-conformance-claims-head] can be made. The W3 states “…if the page does not conform to WCAG only for reasons that are legitimately outside the author’s control then the author can make a claim of partial conformance.” This is an important statement as it both understands that claims achieve no absolute legitimacy for full conformance and that there are legitimate reasons why.
Why I recommend to avoid making a claim