Target multiple landing pages for the same campaign. An iFrame is an object on a page that allows a site to embed HTML from an external source. When the advertiser places the iFrame pixel on the site, 3rd party pixels are displayed within the iFrame allowing a publisher's system to make a direct connection with the end-user. The iFrame is very versatile because it allows multiple 3rd party pixels per affiliate, as well as access to cookies placed by the publisher's tracking system.
An image pixel can be used when the advertiser cannot place an iFrame pixel on the site. Unlike an iFrame, HTML cannot be placed within an image so there are limitations when placing third party pixels when an image pixel is used.
You can only place one third party pixel for each affiliate for a particular campaign, and it must be an image pixel. Any other third party pixels placed, will be attempted from the server level though it isn't guaranteed that the affiliate pixel will track if it needs access to the users' cookies.
Postback URL are also known as Server2Server (S2S) or server side pixels to target multiple landing pages. A postback URL can only fire other postback URLs. Firing a Post Back pixel from a remote server has several advantages over placing a pixel directly on a thank you page. It's a secure way to report sales since it happens in the background. In addition, there isn't any unnecessary HTML on the confirmation page. Affiliates must have the ability to track without cookies. Thus, anytime that you place a postback URL with your advertiser, you will be limited to only placing postback URL's for your affiliates.
Tracking via Hit ID is necessary since cookies won't exist on a server, so the advertiser must be able to process sub ID's to be able to dynamically insert the Hit ID in the pixel when reporting a sale.