=ab is quoted on an exchange, or by a broker. On Eikon you can find the exchanges under page EXCHID01 and following.
To your specific examples:
=RRPS is a special contributor, it is the Thomson Reuters Pricing Service. In other words this is a price delivered by Thomson Reuters (typically a calculated price). Example RIC: JP1YT=RRPS (for more info, see Eikon page RRPSBONDS).
=WD is delivered by the Bond Pricing Agency Malaysia. Example RIC: MYMZ98005A=WD
How to find what it is:
Apart from the above mentioned guides, when looking at the instrument in Eikon you can often find the information, by searching for the source or contributor.