As an online merchant looking to use affiliate marketing networks, do you get to choose your channel?

I am researching online affiliate marketing. Say I am looking to sell something online. Let’s also say I want to leverage affiliate marketing networks to advertise my product. As the merchant, is it common to have the ability to choose which type of advertising channel you are marketed to or is it more like a shotgun blast across the internet?

Or is it the other way around… do the affiliates choose what is eventually advertised by their site?