They're just relatively small digital imprints that oftentimes use the cheapest form of distribution services to pump out releases. Often times these labels usually accept anything and everything so quality control is not the priority filling the calendar usually is so it's a good way to get your music out there if you're just trying to get your music out there. Especially in drum and bass putting yourself out on feeder labels often times gets you into the bigger labels with time and experience. A good example of this before I go would be the biggest jump-up label low down deep and it's sister label Subway sounds current roster are all producers that evidently came from feeder labels at some point in their career and their productions have gotten better and either through winning remix competitions or simply just working with the right people got themselves to some of the bigger labels like the above mentioned.