The buttons on each mixer channel that are labeled as
1-2,
3-4 are the assignment buttons to the 4 recording busses your mixer has.
When you press those buttons down, anything playing through those channels will be feed off to the recording busses if the channel fader is up and the main buss fader is up. This includes previously recorded tape tracks.
And by fixing your "spillage tracks", what you'll need to do is erase and start over again. Once that error has been made, there is no quick fix to undo that. Sort of like having co-joined kids.
The advice you've been offered here if for when you're working with a clean slate on tape.
About the manual, yes. Get one and read it until your eyes bleed! Then read it some more!
Cheers!