Summary
The T-Junction is a junction of two belts by joining one into the side of another. The T-Junction provides utility for the factory design since it provides alternative means to force belt priority.
The proper T-Junction, as seen in the image left, ensures that items on the straight Conveyor Belt (Blue) get flow priority over the one joining into it (Yellow).
It can be used to solve Issues with Hydrogen Priority, ensure saturation of Fractionator Loops, create Sushi Setups that do not jam or just to merge multiple Belts in general.
The Image to the right displays a faulty T-Junction, which will mix belts inconsistently.
