You can file an application to register a trademark in the United States (1) based on a corresponding foreign application or registration to register that same mark for the same goods, (2) your current use of your mark on the goods or services identified in your trademark application which, in the case of goods, must be sold in interstate or foreign commerce and in the case of services must be rendered in commerce that may be regulated by Congress, although offering services via the Internet has been held to constitute use in commerce, or (3) based on your bona fide intent to use your mark on the goods or services identified in your application in the future.
The benefit of filing an application on the latter basis is that it allows you to reserve rights in a trademark before you use that mark in connection with a product or service and, once that mark is in use and you obtain your federal registration, your trademark rights would be deemed to start, not on the date that you first used that mark in connection with that product or service, but rather on the date that you filed your trademark application based on your intent-to-use your mark in the future. However, that application must be based on your “bona fide” intent to use that mark in the future. “Bona Fide” is defined in Merriam-Webster’s dictionary as “made in good faith without fraud or deceit.” See https://www.merriam-webster.com/dictionary/bona%20fide
“Bona fide” intent is important as an Opposer can challenge the registration of your application before the Patent Office’s Trademark Trial and Appeal Board if it can prove that your intent was not “bona fide.” To prove that your intent is “bona fide” you should have documentary evidence of that intent. See M.Z. Berger & Co. v. Swatch AG, 787 F.3d 1368 (Fed. Cir. 2015). So, at the time that you file an intent-to-use trademark application you should have in your possession a business plan, marketing plan, internal correspondence discussing the prospective trademark and the goods and services in connection with which it is intended to be used; if you have simultaneously applied for a domain name, corporate name or LLC name identical to your proposed mark, that is helpful as is the existence of drawings or product prototypes bearing the mark. If you have a proposed brand in mind, you probably have had some prior discussions concerning that name which may be written down or found on an email somewhere. It is important to retain evidence of those disucssions or plans in the event that your bona fides are challenged.