Phone sim cards can be used in other compatible phones.


iPad sim cards can be used in other iPads.


Tablet sim cards can be used in other compatible Tablets.


USB Modem or Wi-Fi Hotspot sim cards can be used in other compatible USB Modems or Wi-Fi Hotspots.


However, you cannot use a phone sim card in an iPad, Tablet, USB Modem or Wi-Fi Hotspot.


Similarly, you cannot use an iPad sim card in a phone, USB Modem or Wi-Fi Hotspot.  AT&T will detect this and block your sim card.