Mithun,
FICA clearing objects are all configurable and clearing that happens is based on certain clearing steps which are configured in SAP.
To Start with, first you need to find the clearing reason through which the clearing has happened.
Clearing reason will give you the process from which the clearing has taken place,
Say for example, 01-Incoming Payments, 08-Auto Clearing, 15-Account maintenance etc.
You will find the clearing reason in the clearing document through T. code FPE3 or in table DFKKOP against the line item (in this Case Final invoice) in the field AUGRD.
Once you have found the clearing reason, then you will find the corresponding clearing variant configured for the particular process.
Clearing in FICA normally gets triggered with any one of the following processes
(a) Incoming payments
(b) Auto-clearing/Standalone Account maintenance.
(c) Account maintenance during invoicing.
The scenario, that you have stated normally happens with option (C) only if you have a PS line item without clearing restriction R.
Now go to SPRO-->Financial Accounting---> Contract Account AR &AP-->Basic Functions--->Open Item Mgmt.-->Clearing Control-->Define Specifications for Clearing Types--->Define Specifications for Invoicing
Check the Clearing variant configured against clearing type R42.
Now go to SPRO-->Financial Accounting---> Contract Account AR &AP-->Basic Functions--->Open Item Mgmt.-->Clearing Control-->Define Clearing variants.
Select the variant that you found out and click on clearing steps.
You will get the Sequence flow/logic as how does the variant work
Now select the Steps and click on display to find out how the grouping and sorting of Open items happens against that step.
This will give you the understanding of the how the clearing variant works.
But ideally the issue here is why there is a payment scheme request line without clearing restriction R, which has triggered the clearing.
Thanks,
Amlan