The Allocator is a block added by Pfaeff's Mod Pack. It functions as a vaccum of sorts. Items placed near the input will be sucked in, and ejected through the output.
A chest or dispenser can be used on either side of the Allocator as an input or an output, as the Allocator itself does not have any storage space. If a block not usable as a storage space is blocking the output, no items will be sucked in through the input.
Unlike a dispenser, ejecting arrows or snowballs will not fire them as a weapon. That is, it will eject arrows and snowballs as ordinary items, as if you dropped them.
The input side of the block is indicated by an arrow pointing towards the input hole, whereas the output is indicated by an arrow pointing away. The allocator is unidirectional and will not transfer items both ways.