RTBlintAlpha
Loss reason 500+: Exchange-specific
What it means
Private values; the spec says to communicate them with buyers beforehand.
The exchange reports this code by expanding the ${AUCTION_LOSS} macro in the bid's loss notice URL (lurl). A bidder that does not set lurl never sees it, and never learns why it lost.
What to do about it
Values of 500 and above are private to the exchange. Only that exchange's integration docs can decode them; ask your partner manager for the mapping.