VAT.

VAT is harmonised within the EU, but there are still significant differences. As the regulatory system is changing continuously as a consequence of, among other things, regulations and legal practice, you have to keep up to date at all times and be aware of which rules are in force. 
A complex system. It's not always easy to know when a business in Sweden is liable for VAT and when you have to register for VAT, i.e. what kinds of transactions are subject to VAT and which costs are deductible.

The complex VAT system means that there are strict demands on you as an entrepreneur, and as an entrepreneur you have to know which reporting rules apply: When does VAT have to be declared? Which exchange rate has to be used? What text has to be specified on the invoice and are there any other requirements in terms of the invoice's appearance and content? 

Issues relating to VAT are also relevant in connection with bigger matters such as sales and acquisitions of assets or businesses, voluntary tax obligation for the letting of premises, reporting and invoicing in connection with the sale of goods and services within and outside the EU, declarations, open declaration in order to avoid tax surcharges in connection with possible doubts about how to report a transaction.