Im sure someone will know al lot more about this than I, but the Access course is made up of units, like assignments which mostly is worth 3 credits.
Like, one of my units were Colour Theory which is worth 3 credits on level 3.
I think most courses are a minimum of 45 credits on level 3 and 15 on level 2.
So I think theyre just asking you to pass the course (60 credits overall) with all distinctions in science relatede units, where two different subjects should be long enough to cover 15-20 credits.
Hopefully someone more knowledgeable will point out if I totally wrong.