When a user adds collateral to their CDP their TRX is held in escrow inside a smart contract for which they are issued PTRX in return. Once staked, a users PTRX holdings can be viewed as a percentage share of the total pool of collateral assets. This model allows the JustStable contracts to a dilute or inflate the size of the pool depending on how well users of the system respond to Liquidation sales, and how wisely voters have chosen to manage the Risk Parameters.
Pooled TRXer can be viewed as a derivative instrument that represents the total amount of Wrapped TRXer that has capitalized the system.