Priority Splitter

There is currently no Priority Splitter a.k.a. overflow splitter in vanilla game. However, it is possible to mimic a priority splitter using various methods.

Uses
A priority splitter has 1 input and 2 outputs, namely Priority Output and Overflow Output. It will always try to fit all input items into Priority Output, and only if the Priority Output is saturated, the items will be sent to the Overflow Output.

Train Station

 * It is possible to create a priority splitter with Trains and Freight Platforms.
 * However this requires Power to function.

Smart Splitters

 * It is possible to create a priority splitter with Smart Splitter with some loop back and utilizing belts of different tiers.

Splitters and mergers
It is possible to create a near-perfect priority splitter with vanilla Splitters and Mergers: The output from the merger is the priority output. The output from the last splitter is the overflow, that can be sent to an AWESOME Sink.
 * Build some foundations as a base.
 * Build 5 Splitters in a row and connect them with belt.
 * Build a second layer of Splitters on top of the first, but these are only for providing a snap points for the Mergers at third layer.
 * Build a third layer of Mergers, all facing the opposite direction. Connect them with belt.
 * Delete the second layer splitters.
 * Connect all Splitters and Mergers with Conveyor Lift.

This is not a perfect priority splitter. The chance of leakage is 1 / (3^n), so in this example it is 1/243. Each additional splitter / merger reduces the leak by two-third of the previous.