a jack multiple aka a "mult" is something that is used with a modular synthesizer to send one source of audio or control to multiple destinations. The passive mult does not require power, does not care what type of signal goes through it, and does not have specific requirements as to what jacks are designated as the inputs.
How To Use 4+4 or 7 way mode
The jack with the white ring is special. If you put something in there, it will connect to the bottom 3 jacks while the top 4 jacks will be on a separate system. if you leave the white ring empty, the top 4 will connect to the bottom 3 as a 7 way mult.