The USCIS issues a variety of notices in response to the filing of an I-129 petition non-immigrant worker form. The principle notice form issued for I-129 petitions is the I-797 Form. There are four primary variations of this form:

Form I-797: Used only for approvals of immigrant visa petitions.

Form I-797A: Issued with tear-off I-94 card, and used for approvals (Notice of Approval) in cases in which the foreign national is present in the U.S. (e.g., received for extension of stay, changes of nonimmigrant status, or change of employer). Most frequently issued for TN visa approvals.

Form I-797B: Issued with tear-off consular notification card, and used for approvals (Notice of Approval) in cases in which the foreign national is outside the U.S. or will use the approval notice to obtain a nonimmigrant visa. Because Canadian citizens do not require a visa to request admission into the U.S., the I-797B form is not used often in TN cases.

Form I-797C: Used for non-approvable actions, such as filing receipts (Notice of Action), request for more information and denials.

See Sample TN Visa  I-797 Form - Approval Notice.