In Microsoft Bing it is the space in an application where ads are displayed.
In Anonymous Ads it is pretty much the same: an advertising space, a source of traffic, that has its own ID and parameters associated with it.
Ad Units are the key elements of our advertising network. Since we don't collect much information about visitors, our targeting is mainly based on ad units. Advertisers select ad units for their advertising campaigns either manually or by choosing the desired sites and categories.
Publishers specify what kinds of ads should not be displayed on their ad units (e. g. they can forbid NSFW or gambling ads, or disallow graphical banners).
If ad unit matches campaigns then it will get money from them and display their ads.
There are 2 types of ad units in Anonymous Ads: ad units of type "Site" and ad units of type "Anonymous".
Ad units of type "Site" have Site URL parameter that specifies the publisher's page with our code embedded (it may be embedded into many pages, but we need to know at least one exact URL to check if it is there).
Our bot visits publisher's page time to time and estimates the probability that our code is there. This estimated probability is denoted as "Health". It may take several days before the estimation becomes accurate.
We may also visit the site manually and set appropriate categories for it if we like it.
We don't know much about ad units of type "Anonymous". They can be embedded anywhere (browser extensions, mobile applications, sites, etc) and they have a description that is supposed to give a hint about origin of traffic. But usually we can't check whether description is correct or not, so anonymous ad units tend to stay in the "Unknown" category forever unless we see a convincing evidence that it should belong to some other category.
CPM for the "Unknown" category is usually much lower than for any other category.