RLGN Quota is usually assigned where your Boarding station is the Source station, alighting at an intermediate station or ur Alighting Station is Destination, boarding at an intermediate. For Eg: say for Mumbai Trivandrum Express, Mumbai to Madgaon or Mumbai to Kozhikode will be RLGN quota. Similarly Madgaon to Trivandrum or Manglore to TVC will be RLGN.

PQWL on the other hand is an intermediate intermediate Quota. A group of intermediate stations form a cluster/pool to form a quota. A cluster In the above example Madgaon to Thrissur will...
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be PQWL.

RLGN/RLWL is never applicable from the source/cluster of stations sharing same quota with the source station.
It always applies on intermediate-end station/cluster of stations sharing same quota from intermediate-that of the terminating stations.
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PQWL...
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is a start-end quota and can be applied to any combination of stations.
Except Rajdhanis, RS (Start-intermediate) is usually merged (In most of the cases; One exception may be Saurashtra Mail). PQWL can mostly be seen for start-intermediate/intermediate-intermediate stations with rare occurence of intermediate-terminating(Group of terminating) stations. Further it can be combined with other quota (Depending upon vacant journey leg) to provide confirmed berths.

I am trying to explain to you so far as my knowledge of these two things goes.
RLGN is Remote Location General quota. If the station where you board the train is an intermediate station and you travel up to a station which is the destination station or close to the destination station, you might be issued tickets in RLGN quota. For example, in 13006 Amritsar-Howrah mail those boarding the train from Varanasi to travel up to Howrah are issued tickets in RlGN quota. The journey leg from Ambala Cant Jn to Amethi also come under the Rlgn quota. But in this train the journey leg from Jalandhar Cantt. Junction to CRJ/Chittaranjan is GN quota because Jalandhar shares the general...
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quota of Amritsar.But PBH/Pratapgarh Junction to HWH/Kolkata Howrah Junction is general quota. ASR/Amritsar Junction to RMU/Rampur is again general quota.
PQWL is on the other hand a quota which can apply to any journey leg.