said by graysonf:
Site-Level Aggregator ID: ?
Whatever you want, but when added to the prefix length the number of bits in it must be less than or equal to 64. I.e. if your prefix length is /64 then this must be 0. If your prefix length is /60 then it must be between 0 and 15 inclusive (0-15 = a 4-bit number. 4 + 60 = 64). This number is added to your prefix to get the subnet for that interface.
The size of the prefix you're requesting from the ISP. Eventually Comcast will support /60-/64 inclusive, but for now only /64 works reliably.