The 6K is the winding resistance, but isn't 100% foolproof in telling you how 'strong' the pickup is. Ceramic magnets have more output than anything else, Alnico II has less output than an Alnico V, and so on. And remember the Seymour Duncan 'Quater Pounder'? Larger diameter magnets had more of a magnetic field, and had more output for the same magnet type and winding resistance. But knowing the winding resistance is still a good, if rough, 'general' guideline as to what you are getting.
Think 6K for Strat-style pickups, and 8K for PAF-style pickups. A 'super distortion' type humbucker might be 12K or more. But again, that's just one part to the puzzle.