Search feeds · also called custom channel, channel, AFS channel
Channel ID (custom channel)
A channel ID is a label attached to search ad requests so that revenue and clicks can be reported separately for each page, campaign or traffic source.
A channel is a tracking label. In Google's Custom Search Ads code, the channel parameter carries an ID created in the AdSense account, and reports can then be broken down by that ID. Several channels can be sent on one request, joined with a plus sign.
For arbitrage, channels are how a buyer finds out what a visitor earned. The traffic platform reports what a campaign cost. Without a matching label on the feed side, revenue would be one lump for the whole account. By assigning a channel to each campaign, country or traffic source, the buyer can compare cost with revenue line by line.
Channels are a limited resource. A feed provider has a finite pool to divide among its publishers, so each publisher gets a fixed allocation and must decide how to use it. Buyers often rotate channels or combine them with a Sub ID and a tracker to get finer detail.
Engines and providers use the same labels for policing. A channel that sends poor-quality traffic can be identified and capped or cut without closing the whole account.
An example
A buyer sends channel "fb_us_roofing" with one campaign and "tb_uk_roofing" with another. The feed report shows $420 against the first and $95 against the second, which can be set against each campaign's spend.
Related terms
Style ID
A style ID is the identifier of a search style saved in an AdSense account that controls how Google search ads and related search units look on a page.
Sub ID
A sub ID is a custom label a publisher passes to a feed or network with each visit, so reported revenue can be broken down by that label.
Tracker (campaign tracking platform)
A tracker is software that records every click an arbitrageur buys, matches it to the revenue it later earns, and reports profit by campaign, ad and keyword.
Keyword-level reporting
Keyword-level reporting breaks down clicks and revenue by the individual keyword or related search term the visitor clicked on the landing page.
Reporting API
A reporting API is a programmatic interface that lets software automatically fetch revenue, cost or performance data from a feed, ad platform or tracker.
Custom Search Ads (CSA)
Custom Search Ads is the Google AdSense code that lets a publisher request Google search ads for a query and display them on its own results page.
Sources: Google Developers: Custom Search Ads reference, Google AdSense Help: Search ads parameter descriptions