A means by which a reader in a passive UHF system identifies tags in the field.

When tags enter the reader's field, they immediately communicate their presence by reflecting back a signal.

This is useful when you want to know everything that is passing a reader; such as, when items are moving quickly on a conveyor.

In other cases, the reader wants to simply find specific tags in a field, in which case it wants to broadcast a signal and have only certain tags respond.