In the "Liabilities" module, I can only select suppliers. Is the creation only for suppliers or does the module also work when I create liabilities like electricity or telephone through it?

For liabilities you need a supplier number (role "Supplier" in the addresses). For telephone costs, for example, you can also create a collective supplier or a separate supplier for each provider. For the accounting export you can also export the supplier addresses or liabilities in the same way as the customer numbers (addresses/account labels).

You can find more information in our manual:

Address

Financial accounting export

Liabilities