I think Bagso means if the bi-lateral is now full then would BHX object or appeal from the point of view of wanting additional flights and/or routes.
I'm not an expert but would suggest if BHX were to do so then it would have to have a carrier 'lined up' to operate a service or at least a stong interest otherwise it's a hypothetical appeal based on a wish, a little like West Ham pushing for extra Champions League spots.
Either way I cannot see it would harm any service to MAN given the lions share of slots are taken up at the 'full' Heathrow.