Dear smart pplz: What on earth does this symbol mean? (schematic)

[ghostsauce]

Over on the upper left, I marked it with a yellow/red box. What the heck is that?

[Tubebass]

Battery

[ghostsauce]

Oh.. Gonna be using the 3pdt/tb wiring scheme we all use, what the heck.. Can I just treat this as the DC+ and omit where it continues to the lug of the input jack?

...wait, yeah, that should be fine cause it's just showing the negative side is grounded at the input I think.

Ok, got it now. Thanks a bunch