If you have a tax rule defined and the criteria is met for the customer i-e country etc, then the subtotal will be including tax and it's called just
subtotal and if it's not then there won't be any tax on that subtotal and in that case it's called
Subtotal(Excl.Tax). Or you can say if you want to apply rule on subtotal but with tax excluding then select
Subtotal(Excl.Tax) and vice versa.