A more thorough explanation:

The I-94 Card is a document that non-immigrant visitors need to fill out when they enter the United States with a visa. It's also called the Arrival-Departure Record or CBP Form I-94. The Department of Homeland Security issues this form to all aliens who are admitted to the U.S. or extending their stay within the U.S.

The I-94 Card shows important information like the date of arrival in the United States, the date when the authorized period of stay expires, and the class of admission. All non-immigrants need an I-94 Form, except for U.S. citizens, resident aliens, aliens with immigrant status, and most Canadian citizens.

I-94s are usually issued at the admission step at the port of entry, by air or sea. The form is attached to the non-immigrant visitor's passport by a Customs and Border Protection officer. Since April of 2013, I-94 Forms have been created electronically upon arrival.

The Form I-94 has two specific perforated sections. The top section is the arrival record and is retained by the Customs and Border Patrol officer at the port of entry. The bottom section of Form I-94 is the departure record and must be retained by the non-immigrant until the time of departure from the United States.

When Maria arrived in the United States from Mexico, she had to fill out an I-94 Card. The Customs and Border Protection officer attached the form to her passport. Maria made sure to keep the bottom section of the form, which is the departure record, until she left the United States.

Another example is when John, a student from China, entered the United States with a student visa. He was required to fill out an I-94 Card, which showed the date of his arrival and the date when his authorized period of stay expires. John kept the top section of the form, which is the arrival record, and gave the bottom section to the Customs and Border Patrol officer.

These examples illustrate how non-immigrant visitors need to fill out an I-94 Card when entering the United States with a visa. The form shows important information about their stay in the country and must be retained until the time of departure.