Just to make sure I'm understanding right: are you saying you get a visa and can enter for the following 90 days, and then you can stay an additional 90 days (assuming extension) AFTER you enter? Or you can only stay for 90 days in total after the date of issue on the visa?
For example, if you got a visa on August 1st, you could enter until around November 1st. But if you entered on October 1st, would you still have to leave on November 1st (around 30 days)? Or could you stay for 60 days + extension until around January 1st (around 90 days)?