Double entry means 2 visits within the alloted duration or two entries of equal duration?

I've applied for an F because I have a 3 month internship. There may be a chance of extension, so I'm wondering if my double entry, 90 day F lets me enter china twice within 90 days or I can enter twice, the 90 days counting down from the first entry and then restarting upon the second. I was confused because I originally tried to apply for 120 days but the clerk said in that case I would put "60" on the form and have two entries....