I have to confess I don't understand why they are referring to this as 'the butt edge', since AFAIK shingles don't 'butt' (vertically) but rather overlap each other; however, this is clearly a US document, so perhaps this is just a usage I'm not familiar with. I suppose one might also imagine a possible error for 'bott. edge' (i.e. 'bottom'). Either way, this is the only way I think it makes sense in the installation context you've explained.