bc40ton

I will be running some 1/0 gauge cable for a 2kw inverter after installing a second battery. I have the new cable laid out and bent ready to install when I see a positive cable running from the battery marked #1, that goes along the firewall above the engine (5.3) and over to the back of the fuse panel. But I cannot find any info on this cable as to what it does or if it is used to power the fuse box because it looks like one side of the connection in the second pic turns and goes down maybe to feed the fuse box. Any help would be appreciated. Here's a pic. I looked up the part # of the wire and it says "positive cable" #23119639