${AUCTION_IMP_ID}

What it expands to

ID of the impression just won, from imp.id.

Expands in notice URLs and adm with the imp.id of the impression the bid won.

What to watch for

On multi-impression requests this is the only macro that says which slot cleared. Bidders that assume one imp per request tend to drop it and then cannot attribute wins per slot.

Encoding

When a macro rides inside another URL as a query parameter, the expanded value must survive URL parsing: exchanges support suffixed forms like ${AUCTION_IMP_ID:B64} (base64) where documented, and values containing reserved characters must be percent-encoded. An unencoded expansion is the most common reason a notice URL truncates at the first ampersand.

Related

• All substitution macros
• Where nurl, burl, and lurl fire in the delivery flow
• Loss reason codes
• ${AUCTION_ID}
• ${AUCTION_BID_ID}
• ${AUCTION_SEAT_ID}
• ${AUCTION_AD_ID}
• ${AUCTION_PRICE}