Disc Allocation
At one time, on digital audio workstations, only certain types of disc drives like SCSI (Small Computer System Interface), firewire (IEEE 1394), or magneto optical were allowable. All other devices were simply too slow.
As transfer rates improved, most digital audio workstations can now record and playback from a variety of drives (internal and external, fixed and removable), as well as from compact flash, SD cards, and even some USB sticks, and they can do this at sample rates and bit depths much higher than used by earlier workstations.
Often with sessions at higher sample rates, it's recommended that the files be allocated to multiple devices, to better handle the data, as opposed to trying to playback all the tracks from one drive.
Disc allocation is a way to manage what device is supplying the data for what track. Here, certain tracks can be assigned to certain media devices, or the tracks can use what's called a round-robin allocation, meaning that the data will automatically be distributed evenly to all usable devices connected to the DAW.
Dividing the data this way approximates a RAID-0 condition, where the data is spread across multiple drives, allowing faster access. However, care needs to be taken not to unmounts or remove any of the drives that contain the audio (and possibly video) files, so that the session won't be missing certain tracks.