Olivier,
Here are the links to the relevant pages of SAP Library:
GR/IR Clearing Account - Closing and Reporting (FI) - SAP Library
Posting Acquisition Tax - Closing and Reporting (FI) - SAP Library
Regrouping the GR/IR Clearing Account - Closing and Reporting (FI) - SAP Library
I also recommend that you read the program documentation
Regards
Th.