On base amount: The VAT will be calculated based on the linked VAT percentage.
Fixed amount: In general, you fill in the VAT directly on the VAT return form. This might be the VAT to be reported if you use your car for private purposes, for example.
Reverse charge: In the case of reverse charge VAT, the VAT is shifted to the customer or supplier, but for tax reasons you need to register and account for the VAT yourself as well. The VAT will be calculated on the base amount at the reverse charge VAT percentage. As the entries are processed, two additional lines are recorded: one to register the VAT (categories 2A, 4A and 4B), and one to reclaim the VAT (category 5B).