Suitability is the requirement that a recommendation be appropriate for the specific client, based on their circumstances, objectives, and risk profile — and that the basis for that judgement be documented.
The test is specific rather than general. A strategy that is entirely reasonable for one client can be unsuitable for another with different obligations, time horizon, or capacity.
Suitability is proved by documentation. What was known about the client, what was recommended, and why it followed from the first.
Advisers who have been through an examination generally describe the same lesson: the reasoning existed, but it existed in their head rather than in the file.
For investment advisers, suitability sits inside a broader fiduciary duty. For broker-dealer representatives, Regulation Best Interest replaced the older suitability rule with a more demanding standard.
Circumstances change. A recommendation appropriate at the time can become inappropriate, and the obligation to review is ongoing rather than satisfied at onboarding.
This is where capacity analysis earns its place — a change in the plan changes what is suitable, whether or not the client's disposition changed.