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"ATL" stands for "Above The Line", meaning that the advertising is going to be deployed around a wider target audience, e.g. television (TVC), radio, or billboards. applicable when a product is directed for a broader spectrum of consumers, for example a soft drink company might contract an ATL advertising agency to develop ads targeting a broad audience.
"BTL", or "Below The Line", suggests that the advertising is going to target a specific group of potential consumers. BTL advertising agencies will be hired to help companies to develop ads and promotion strategies in a creative way, directed to certain groups of, using tools like direct emailing, or direct product demonstrations for a specific group of people, like giving away vitamin samples at the door of a famous gym.