Content Exclusions, formerly Site Category Exclusions, is less about placement targeting and more about Brand Safety.
Here Advertisers can identify types of content (eg. tragedy, crime, sexually suggestive etc) that they do not want their Brand associated with – and opt out of them all at the Campaign level.
We recommend opting out of all available Content Exclusions as a matter of best practise, with the obvious exception if your Product/Service can serve that area.