RLWL is Remote Location Waiting List, Running Line Waiting List or Road-Side Location Waiting List. It is generally meant for passengers passengers travelling from intermediate stations up to or short of the terminating station. RL quota tickets are never available from the originating station or source station of the route.
GNWL is General Waiting List. It is meant for passengers travelling from the source station or from stations near the source station upto the destination station or near the destination station. This is an end to end quota and this quota is used by passengers travelling from source station or stations near the source station to the destination station or stations near the destination station of the train.
PQWL...
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is Pooled Quota Waiting List. The word pool as a verb means to collect money, information etc from different people so that it can be used by all of them. In Indian Railway too some berths from different quotas are collected to form this pooled quota and this quota can be used by any journey leg, meaning (i) source to intermediate,(ii) source to destination, (ii) intermediate to intermediate and (iii) intermediate to destination.
Finally I want to say that merging of these quotas is often observed in different trains so that if you search through the PNR forum you will find a quota missing from a train. Actually in that train that quota has been merged with another quota. But every quota is present in every train.