The Buffer is a very simple sorting machine, very similar to a Chest with 5 separate compartments. When providing items to the buffer via Pneumatic Tubes, Pipes, or similar devices, each of the 5 input sides will fill a different compartment. For the Buffer from Immbis Core & TubeStuff mods, see here.
The Input and OutputSorting Machine
A common application for this block is in situations where different inputs provide their materials at different rates.
A regular chest could easily be filled up by stacks of single items, leaving no space for other resources.
The GUI for the Buffer shows 5 columns. These correspond to input sides of the machine.