TM means "trademark", while SM denotes "service mark". Trademarks are primarily used on products (i.e. goods), while service marks are employed to label services. Both TM and SM can be used prior to registration with the federal trademark office (USPTO). Once a trademark or a service mark is registered, the (R) in a circle is used to indicate that the trademark has been registered. The use of the symbols TM (trademark for goods) and SM (service mark for services) is recommended to claim the rights on unregistered marks that have not been used by others in commerce.

References:
[1] Stim, R. "Intellectual Property. Patents, Trademarks, and Copyrights" West Legal Studies.
[2] Black's Law Dictionary 5th ed., (West Publishing, 1979).

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