Final answer:
In IEEE 802.11, the address flag used in a frame when going from one AP to another AP in a WDS is the 'To Distribution System' flag, indicating the frame's movement towards the distribution system.
Step-by-step explanation:
In IEEE 802.11, which is a set of standards for wireless local area networking, frames that are transmitted between APs (Access Points) in a Wireless Distribution System (WDS) must contain multiple address fields in the frame header to accommodate the source and destination addresses along with the addresses of the transmitting and receiving APs. In the scenario where a frame is moving from one AP to another AP, the address flag typically used in the frame header is the "To Distribution System" (DS) flag.
This flag is used to indicate that the frame is heading towards the distribution system from an STA (station), or in this case, an AP acting as an STA, and typically has a value of '1' when the frame is going to the distribution system, and '0' when the frame is coming from the distribution system.