Hi,
I try to use the VAT feature on Kmymoney but I don't understand how it works.
Do I need to create two new categories (VAT in, VAT out) and then edit the other categories ?
If I choose "allow automatic VAT assignment", I can't see the newly made category VAT in or VAT out ?
Please can you help me ?
Thanks.

Looking at the user manual it seems that you are right.

In the first place you need the VAT categories (VAT in, VAT out as you've said) and for these you need to check the 'VAT category' on the Tax tab.

Secondly you need to go trough the categories that involve VAT transactions and check 'Enable auto VAT assignment' on the Tax tab of these categories.

Now when you will enter a transaction in these categories the transaction should be split into a net split from the account to the category and a tax split from the account to the VAT category (in case of an expense and the other way around in case of an income).

Hope that helps.