Yahoo determines that on the basis of the domain name. And also, they can change over time - I've had 1-clicks go to 2-clicks, and 2-clicks go to 1-click. Moreover, there are some who swear that 2-clicks get a better earning per click (I'm testing that now). Your account manager can change a 1-click into a 2-click, but he can't change a 2-click into a 1-click.