A 'multi' or 'multiple' in modular parlance is a series of jacks connected together to distribute one output to several inputs.
Passive means that the signal is not buffered, and therefore works both ways: any jack in a passive multi can be either an input or an output.
In the context of Logoi, and same with Stoicheia, either one of the two bottom side jacks can be used as the input.
This is so that you can daisy-chain your master clock through-out your system without having to use a separate multi, or stackable cables, or other forms of signal distribution tricks. Simply plug your clock into one side of the module, and use the other side as an output.