The explanation for SID has been just copied, but the meaning is same. On Maretron document they write:
SID - The sequence identifier field is used to tie related PGNs together. For example, the GPS200 will transmit identical SIDs for 126992 (System Time), 128259 (Speed), 129026 (COG and SOG, Rapid Update), 129029 (GNSS Position Data), 129539 (GNSS DOPs), and 129540 (GNSS Satellites in View) to indicate that the readings are linked together (i.e., the data from each PGN was taken at the same time although they are reported at slightly different times).
Will that explain it better?
So it could be same for DC information. You can provide DC status and battery status information with same SID, if they are measured at same time. Due to transmission delay there will be small difference, when messages will be received, but for same SID receiver can then use same time for values e.g. for calculations.