The msdn documentation was a little hazy on this subject. Here it is as I understand it….

Lets start from the beginning; and adrotator will take a list of images and rotate through them every time the page loads. Some ads may need to appear more often than others. That is where the Impressions property comes in. It allows to give some ads more weight than others. Perhaps one person is paying more for the ad.

OK, so how do we use it? if every image should be weighted equally just set it to 1. Other wise you need to figure out the ratio. To figure it out you add up all the impression values in the file. Each ads ratio is it’s impression value divided by the total.

So if there were three ads in the file and they looked like this:

ad 1 imressions = 1
ad 2 impressions=2
ad 2 impressions =2

The total is 5. Ad 1 will be show 1 out of 5 times. Ad 2 will be shown 2 out of 5 times and so on.

I like to work in percentages. To do this just divy up the ads until we get to 100%.

Ad 1 impressions = 20%
Ad 2 impressions= 40%
ad 3 impressions= 40%

Just make sure they all ad up to 100 and your done.  The percentage is the number of impressions.  1, 2,2 is actually the same as 20, 40, 40 so why not base it on 100 so it’s easier to figure out?

Good Luck.

P.S. I saw a blog somewhere that wrapped an adrotator in a UpdatePanel that pdated on a timer. Interesting. Quicker than flash and more flexible than an animated gif. I’ll be filing that in the back of my mind.