A new accounting category can be created in the admin area under the menu item
Accounting categories

Assign a name of your choice
Whether it concerns invoices or credit note
A posting category number (if you want to transfer the invoices to DATEV, for example).
the possible tax rates and
The business unit for which the category is to count.

The posting categories are used by Accounting to separate revenues and expenditures and to be able to valuate them later.
For example, the individual categories can be evaluated when they are transferred to Xero.

When you write invoices, you can then assign one of these posting categories to each individual invoice item.

It is completely up to you whether and how detailed you define these categories.
Examples of posting categories:

If you have not defined a accounting category or accoungt customer number, Keyline will also inform you: