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There are some occasions when an ACK is not used. For instance in an RTS/CTS frame exchange, the CTS frame serves as the acknowledgement. Also a QoS No ACK policy might be used in some QoS Data frames. In most cases, unicast frames must be a acknowledged by an ACK or Block ACK frame.

From

Cover of CWAP® Certified Wireless Analysis Professional: Official Study Guide

Note

NO acks