[![T-S plot of a vapour compression refrigeration cycle][1]][1]


  [1]: https://i.sstatic.net/PoCPF.gif


My question here is, why do we need point 2 to be on the saturated liquid line, i.e. why do we need a saturated liquid for the throttling process? Why can we not use it in the liquid region or subcooled region?

Similarly, why do we need a saturated vapor to enter the compressor (point 4)?