This is the distinction that matters most, and it is the one most often blurred in contractor marketing. A contractor installs to a specification. Determining what the specification should be, and whether a finished lot satisfies applicable law, is a different function performed by different professionals.
Depending on the project, the people involved besides the contractor may include a Certified Access Specialist, an accessibility consultant, an architect or engineer, legal counsel, and the local building or planning authority reviewing permits. On a small restriping job none of them may be needed. On a lot being reconfigured, or one already the subject of a complaint or demand letter, several usually are.
A contractor who states plainly that they install to a layout but do not certify compliance is describing the arrangement accurately. Treat a guarantee of compliance from the company performing the physical work as a reason to ask more questions, not fewer.