Stamp

Maybe I’m wading in here more on impression than deep study of it, but I think a seal is saying “I am qualified by education, preparation, experience, and (most likely) testing to do this plan” (for any sheet it is on- sometimes as required by reviewing jurisdiction). That also implies it is your property. But doing the work in the first place gives you the copyright unless you have consented to release it to the client, like ads I have seen asking for a transfer. I presume that is because they are real estate people who want to re-publish the work although they could just get your permission for that anyway. As far as insurance, if you can be tracked down, you may be liable even if you don’t seal it, so it’s probably wise to add the comment “not for construction” or equivalent if the drawing is only a concept.