thanks in advance = looking for a mech for an old bike and though I've fitted a few front mechs in my time only just realised how potentially complicated it can be - maybe I was just lucky before
It's pretty meaningless in the the context that they have used it because they don't give it a value.
It is, I believe, the included angle between the chainstay and the seat tube and defines how far you can lower the mech before the rear of the cage hits the chainstay. Ideally your chainstay angle should correspond to that which Shimano quote. A larger angle than recommended will mean that you can probably operate with a smaller outer chainwheel than that recommended by Shimano,
