A Restriction Tube is a tube from the RedPower 2 mod. This tube assists in the management of pipe overflow. Just like the pipe's name, it restricts items from entering its input and going to its output only if there is no other place for an item to go; e.g. all other chests are full.
Using As An Overflow
If a tubing system has the option of overflowing; Redpower2 Pneumatic Tubes have the ability of allowing items to circle the system to allow for a 'vacancy' to appear where the item will flow to.
However, a Restriction Tube could be used to filter out the excess level of items and send it to a secondary area; as these tubes will only allow input if items cannot go anywhere else. Note that these do not work in reverse as they only have one input and one output.
Logic Behind The Pipe
It is suggested by many players that the reason this is always the last option for items in a Pneumatic Tube system is by telling the Redpower2 system and machines that the Restriction tube is actually approximately 5000 tubes long. This suggests to the system that it should find the shortest option available as very few players are likely to have a system exceeding 5000 tubes in length.
It can be used advantageously to prioritize outputs. Stacking up the restriction tubes can suggest for the items to go to the output with only one Restriction tube instead of those with 3 or 4. This can be said to be advantageous, but color coding Pneumatic Tubes is a better idea.