Wiring diagrams

From the wiring diagrams how do we determine what the abbreviations on modules mean ? Does a + symbol mean its a 5v reference source . am working on a 2006 ml350 and have mithcell 1 dyi diagram.

to my knowledge i have never see refrence wire with (+)simbol,This simbol indicate battrie voltage after the ingnation is turn on, i havnt see a spefic simbol for 5 volt or refrence,

but on most of the american cars it shows either refrence or 5 volt