Tab, stylized as TaB, is a diet cola soft drink produced by the Coca-Cola Company, first introduced in 1963. The soda was created by Coca-Cola after the successful sales and marketing of Diet Rite cola, owned by The Royal Crown Company; previously, Diet Rite had been the only sugarless soda on the market. Tab was “marketed to consumers who want to keep ‘tabs’ on their weight.”
The soda was fairly popular throughout the 1960s and 1970s, and the Coca-Cola Company made several variations of it, including Tab Clear and Tab X-Tra, as well ascaffeine-free versions.
Info gleaned from Wikipedia