Box 3 is seen by many as a type of net-wealth taxation. For Box 3 your assets are deemed to generate a fictional percentage annual return on investment. This fictional income is regardless of actual investment income or losses. The fictional income revenue is calculated over the net sum of your assets and as of tax year 2017 is progressive. You can see the Box 3 tax table here.
Starting 2011, a reference date of January 1st of each fiscal year is used to establish the net sum of your assets. For fiscal years before 2011, an average of the net sum of your assets on the reference dates of January 1st and December 31st of each fiscal year is used for the Box 3 calculation.