[css3-content] ::outside::outside vs ::outside(2)

I understand (because the specification explains it very clearly) why it is necessary to have the special syntax of ::before(n); because ::before::before and ::before(2) are different. 

However, I am not clear why special syntax of ::outside(n) is needed because I can't see any difference between ::outside::outside and ::outsize(2). The specification does not explicitly prohibit ::outside::outside (so I presume that it is valid) but also does not mention it either.

Is ::outside::outside allowed?
How does it differ from ::outside(2)?