Hi Sriram,
The basic concept behind use of PC (not only 20, but all) is, to group wagetypes based on behaviour and treat them accordingly, rather than based on the key.
All processed wagetypes are queried on the current value of requested PC and according to that value, different actions can be performed.
PC20 usually is not used frequently, but on the contrary only once. It is supposed to controll the final treatment of all remaining wagetypes at the end of gross calculation - so don't mess with it.
PCR are strictly Position dependant. Each blank in any operation-statement has meaning and cannot be left out.
Make sure to read the F1-documentation on frequently used operations, like ADDWT, AMT and the likes and learn the positioning by heart. It will save you pains in the future.
Because once upon a time, when SAP was still young, memory was expansive and so the Limit for each operation, including all parameters was set to 10 characters. That lead to a rather restrictive use of cryptic, short names and multiple use, differing even on the position of blanks. That is the reason, why we still have to obey the positioning rules.
Best regards - Jörg