Search engine found this.
First 10 rows . . .
AfterHoursChangeRealtime After Hours Change (Realtime) c8
AnnualizedGain Annualized Gain g3
Ask Ask a0
AskRealtime Ask (Realtime) b2
AskSize Ask Size a5
AverageDailyVolume Average Daily Volume a2
Bid Bid b0
BidRealtime Bid (Realtime) b3
BidSize Bid Size b6
BookValuePerShare Book Value Per Share b4
If Yahoo! doesn't document their API, you'll probably have to discover the tags by reading a lot of newsgroup posts or by reading a lot of source code. The YDN forums seem suspiciously silent when it comes to question about the finance API, and they don't show a finance API in their YDN index.