Mechanical excursion is how far the cone can actually move before the surround/spider rips or just stops cone motion. It's referred to usually as xmech.

Xmax is one-way *linear* excursion. It used to be defined as voice coil height - gap height/2. These days, companies measure it all different ways with all the new "long gap, longer coil" woofers coming out. Eminence has at least stayed tried and true and measures xmax by voice coil overhang. So you'll have a true, linear xmax of 3mm plus a bit of non-linear play as well.

Adam