There are two different types of I-797, Notice of Action, that an employer may accept for Form I-9:
I-797, Notice of Action, receipt notice of I-90, Application to Replace Permanent Resident Card, with an expired PRC. This notice extends the validity of the PRC for 12 months from the "Card Expires" date on the front of the PRC. This combination of documents is an acceptable List A document that establishes identity and employment authorization and does not require reverification.
I-797, Notice of Action, receipt notice for I-751 Petition, to Remove Conditions, with an expired PRC. This notice shows that USCIS has extended the PRC validity. This combination of documents of I-797 and PRC is an acceptable List C document that establishes employment authorization and must be presented with a List B document that establishes identity for new hire. Employers must reverify the employee's employment authorization before the extension ends.