At the USPTO, when you apply for a trademark registration, the USPTO first examines the trademark application for formalities and substantive issues (such as likelihood of confusion). Only if the applicant overcomes any USPTO objections, will the application be published and a third-party can oppose the application.
The procedure in Mexico is different. It is a parallel system. The application shortly after it is filed will be published and a third-party can oppose. Simultaneously, the Mexican Industrial Property Office (IMPI) will be reviewing the application for formalities and substantive issues -- and the IMPI has 6 months from filing date to issue a Office Action.
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