Can any one explain me what is the use of PARAMETER_SET_CONTEXT_ID

Answer1
Each parameter (field) in ITG has a unique identifier. PARAMETER_SET_CONTEXT_ID is used to identified each field in the request. So, when using rules, you can make sure of a parameter using its individual identifier (PARAMETER_SET_CONTEXT_ID). It can be used for several other reasons.

Now, I use this PARAMETER_SET_CONTEXT_ID mostly when using a Table Component. You can refer to PARAMETER_SET_FIELD_ID too, it is very similar. I hope this asnwers your question. Let me know if you want something more specific.

Answer2
PARAMETER_SET_CONTEXT_ID is a primary key from the table KNTA_PARAMETER_SET_CONTEXTS, it is linked to table KNTA_PARAMETER_SETS using PARAMETER_SET_ID. PARAMETER_SET_ID indicates whether the data item is "Request Header Fields", "Workflow Commands " etc.
Based on PARAMETER_SET_CONTEXT_ID , you can find,say, field information for a req type from KCRT_REQUEST_TYPE_FIELDS_V, which stores field name, parameter no, token its validation etc.You can find more info from the Technical Ref Manual from Mercury.