From Sylvia Boecker: 8 CFR 212.15(n) states: (4) Expiration of certificate or certified statement. The individual's certification or certified statement must be used for any admission into the United States, change of status within the United States, or adjustment of status within 5 years of the date that it is issued. To me, that means that the TN nurse who lives in Windsor and commutes each day to Detroit to work, has a Visa Screen certificate which is "used" for an admission into the United States. I interpret that as her being able to use it indefinitely, not for only 5 years.

From William Stock: Sylvia's reading is arguably correct, except that it also leads to the conclusion that once the certificate is "used" it is "used up" and a new certificate would be required for each entry. I think a better reading is that any certificate may only be used for admission for five years after it is issued, i.e., on the date of admission, it may only be "used" for that admission if it is "within 5 years of the date that it is issued."