The Relay will automatically output any stacks of items placed into it. Any Buildcraft machine designed to place items into an adjacent chest will treat relays as chests (Industrialcraft machines like the Pump and Miner may not recognise it). This makes it simple to connect a non-redpower machine, such as an Automatic Crafting Table, to your tube network. It also makes an excellent input chest for an automatic Sorting Machine.
To remove items from a non-redpower machine, make sure the larger hole is pointed towards the machine.
The Relay has a 3 x 3 inventory which can be accessed by right-clicking. When placed inline on a tube system items will pass through the Relay unless a valid destination becomes unavailable. This can be useful on a tube connected to a macerator where different ores that do not stack together can wait until the macerator catches up.