A more thorough explanation:

An opinion letter is a written document that provides an expert's understanding of the law as applied to assumed facts. It can be prepared by a lawyer or an accountant and is often requested by a client. The purpose of an opinion letter is to provide advice and guidance to the client on a specific matter.

For example, a lawyer may provide a title opinion to a client who is purchasing a piece of real estate. The opinion would state whether the title is clear and marketable or whether it is encumbered. Another example is a comfort opinion, which is provided by an attorney to reassure parties that a registration statement complies with securities laws.

Opinion letters can also be used in legal proceedings. A court's written statement explaining its decision in a given case is called an opinion. A dissenting opinion is an opinion by one or more judges who disagree with the decision reached by the majority. An advisory opinion is a nonbinding statement by a court of its interpretation of the law on a matter submitted for that purpose.

Overall, an opinion letter is a formal expression of judgment or advice based on an expert's special knowledge. It can be used in various contexts, including legal proceedings and business transactions.