You can turn an expense/income category into a VAT category, which means that it receives all the splits that actually make up the VAT payment towards the government. You can also enter a specific percentage rate.
Categories can be assigned a VAT category, which allows KMyMoney to split a transaction for a category into two parts, one for the category and one for the VAT. Depending on the setting of the gross/net amount switch, the amount you enter for the transaction is the gross or net amount.
Example: In Germany, there are three VAT percentages (0%, 7%, and 19%). So I have a main category "VAT paid" and three subcategories, one for each percentage rate. I create these on both the income and the expense sides so that I have six categories in total. For the goods I buy, I select one of the above mentioned categories as the "VAT category assignment".
When I buy goods, I enter them into a transaction. Let's assume I have selected the gross amount entry method, once I enter the category and amount, KMyMoney will recognize that there's a VAT assignment and calculate the VAT part, create a second split with the VAT account and VAT amount and reduce the category amount.
The same applies to income categories but the other way around. Hope that makes sense.