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Urgent Pointer is used only when the URG flag is set. The 16-bit number is added to the Sequence Number to indicate the end of the urgent data.

From

Cover of Routing TCP/IP, Volume 1, 2/e

Note

When is the Urgent Pointer field used in the TCP header? #TCP url:https://www.safaribooksonline.com/library/view/routing-tcpip-volume/1587052024/graphics/01fig17.gif