DFA stands for Doubleclik For Avertisers and is the long time leading ad serving solutions for advertisers that has been bought by Google.
The term "For Advertisers" is a little misleading as DFA is mainly used by agencies.
DFA offers classic agency ad server features and also media planning features.
DFA belongs to Doubleclick’s suite that claims to serve more than 20 billion ad impressions per day.
An example of planning with DFA: