This is a new method we will use to load random banners. There are a small, fixed number of "slots", currently 30, which represent the different possible responses when a given target audience requests a banner. For example, if an anonymous user is retrieving Spanish wikipedia content from New Zealand, and two banners of equal weight are available to serve, then slots 1-15 are banner one, and slots 16-30 are banner two.
The fixed number of slots introduces an error in banner allocation -- if there are many different banners for a target audience, the probabilities of delivering each will not be perfectly related to their relative weights, but will be rounded to the nearest whole number of slots. With 30 slots, this rounding is at multiples of 6.7%, so with four banners of equal weight, the allocations will be 26.7%, 26.7%, 26.7%, and 20%.