I should know this with the amount of bikes I have but what are full length outers ?

Is it fitting another length of outer where there wasn’t one as standard ie on the gap on the top tube?

Yeah but in one continuous piece of outer.

it’s a continuous run of outer from shifter to near the rear mech.

The cable guides tend to be ‘drilled out’ to allow this. It’s not just fitting an extra length on the top tube but having a joint at each end.

Much better for British winters too.