On my jap wrx I can seem to work out why that little pipe off the header runs down the radiator to the battery and has the coolent level on it

I had a pipe split on me and I filled it back up.. but which do i fill up

the header tank, or take the yellow top off the other bit and fill that up,

sorry for the poor explanation but its friday and have other things on mind

 

Both, drive it until it warms up fully, let it cool, check and top up as needed after any air bubbles have worked their way out, repeat until it's still full when you check it.

 

Concentrate mainly on the header tank though...