This EXT6+ 3km range Remote Trainer comes with one active collar but by buying suitable collars you can add up to six collars as required. It enables the handler simply and accurately to address his dogs with an appropriate correction, according to his misdemeanour, without the dog realising that the correction is coming from his collar. The dog needs to think that the tone/vibration or level of correction with his trainer’s voice command or whistle is warning him of something “out there” which he needs to avoid, the correction is startling enough to get his attention and make him return to the handler for praise and attention.