Ojo con la ortografía - es "below"
Below the line (traducido literalmente al castellano significa debajo de la línea) más conocido por su acrónimo BTL, es una técnica de marketing consistente en el empleo de formas de comunicación no masivas dirigidas a segmentos específicos La promoción de productos o servicios se lleva a cabo mediante acciones se caracterizan por el empleo de altas dosis de creatividad, sorpresa y sentido de oportunidad, creándose novedosos canales para comunicar mensajes publicitarios.
BTL es también conocido como "marketing de guerrilla" en algunos países latinoamericanos. (Wikipedia)

The terms 'below-the-line' promotion or communication, refers to forms of non-media communication, even non-media advertising. Below-the-line promotions are becoming increasingly important within the communications mix of many companies, not only those involved in FMCG products, but also for industrial goods.

Some of the ways by which companies do BTL (below-the-line) promotions are by exhibitions, sponsorship activities, public relations and sales promotions, like giving freebies with goods, trade discounts given to dealers and customers, reduced price offers on products, giving coupons which can be redeemed later, etc.

In Spain, promotional activities that do not use mass media such as television or press to communicate with clients are known as below the line or btl. Above the line refers to more conventional activities, through the line to activities including atl and btl. In a previous incarnation I used to handle through the line communications for major companies in Spain.

Mira este link.

Saludos y espero sea de utilidad.