The Salesforce Audience Studio Control Tag is a snippet of code associated to a confid that gets generated to include in Websites, Media and uniqueids (confid) for mobile SDKs.
Similar to Google Analytics for websites, Control Tag is a snippet of code that gets placed in the </head> section of a website to enable tracking, targeting and more. Including, but not limited to the following...
Data Capture - User Matching - find out more about User Matching and Match Rates
Data Capture - Attributes - additional contextual and user information about site visitors often using a DataLayer
Data Capture - URLs - URLs are captured to emulate data leakage and provide additional url-related options in segment building
Tag Management - SuperTag delivery of tags - Optionally use the Tag Management features to deliver 3rd party code for tracking or targeting and additionally leverage advanced rules-based decisioning for where and when to deliver the code. More about SuperTag Tag Management - Guide, FAQ
Real-time segments and site Personalization - target first time visitors and personalize site content in real-time using real-time segments that are calculated in the browser. Activate with real-time activation partners to suppress or retarget offsite in real-time. More about real-time segments.
Owned and Operated (O&O) Targeting - Use 'Send to Site' activation features combined with O&O targeting to send segments directly to your ad server or CMS. More about O&O.
Find out more about related questions in the following articles.