Bloomberg provides PX_BID and PX_ASK on a daily basis, but it's not clear exactly where these numbers come from. Are they closing bid and ask prices, or are they averaged over the entire day? For stocks, are they pulled directly from Nasdaq/NYSE dealers/specialists? Also, if anybody knows, were they pulled from the exchanges even before Regulation NMS?
PX_BID and PX_ASK are the static equivalents of BID and ASK, the latter two of which populate in "real time" (i.e. as they are dynamically updated). So the PX_BID and PX_ASK values are dependent upon when you pulled the data.
Bloomberg's source depends on the asset in question and the exchange on which they are listed, but the data does come from the exchange, an associated MM, or an ECN initially. I'm not sure about data pre-Reg NMS.
Perhaps a sidebar, but something that might be of interest: because there is less incentive to reduce publishing delays for trades relative to quotes (i.e. since trade delay allows transient post-hedge advantages for the traders, while old quotes are penalized), trades are more stale than quotes. This could account for unusual PX values for last trade vs. the quotes. However, propagation delays in quotes can also be significant, often artificially reducing the NBBO given a delay-induced offsetting effect, which sometimes shows a locked or crossed market. This should also be considered when pulling quotes.
You might find Rosenthal (2011) useful, as well, if you are interested.
They represent the current
If you look up those fields in the terminal
They are refreshed each time you query them. They come from the NBBO quote at the time you query them.