DFC
Mmmmm... Wasn't trying to get into a discussion of what should go in the purpose field. It's normally decided by the sponsor, in this case it will have been CAA/DAP/AUS who have a set of standard templates they use which have been drawn up in consultation with NATS/AIS. ICAO Doc 8126 will tell you what should be applied to a particular NOTAM Code. In this example I don't have a copy of 8126 to hand but I'm fairly sure that it will tell you to use M for QWALW and BO (Operationally significant and for inclusion in PIB) for QRTCA.
I'm told by AIS that they very occaisonally change the purpose field but try not to as it generates queries from EAD and other NOTAM offices as it doesn't comply with the expected entry for the activity. A more acceptable fiddle is to use a different Q code. For example Taxiways are M while runways are BO, so giving the high speed taxiway a runway code will ensure it's included in the PIB. This is a pretty acedemic discussion if you're using the AIS site because as I mentioned earlier you'll get Misc nOTAM anyway. Others may be different.
Like you I find the AIS site fine but I appreciate that many others prefer using something else.