That specification means about whatever you want it to mean. Speaking broadly, a mid-weight" backpacking boot is one designed to carry moderate overnight loads (3035 pounds) on smooth and/or rough trails. Its not meant to be worn as a mountaineering boot or for extended off-trail use.
All of this is sort of meaningless from a practical standpoint. For some people a mid-weight boot wont be beefy enough, for others it may be more boot than they want. Its really a matter of how rugged your ankles are, and how light you like to travel. Myself, I prefer a little sturdier boot, both to accommodate off-trail jaunts, and to protect a right ankle thats prone to roll on me.
In any event, Vasque has discontinued the Skywalk. Probably the Vasque Wasatch GTX ($160; vasque.com) would handle the same sorts of trails and loads. Its an all-leather boot with a Vibram sole and a Gore-Tex bootie. Similar boots from different makers would include the La Sportiva Trango Trek ($170; sportiva.com) and Montrail Torre GTX ($160; montrail.com). Id wear a boot like this when carrying up to 35 pound loads on hikes where I dont intend to do much bushwhacking when fully loaded, but I might do some scrambles with a daypack.
In the Vasque line, a step up to a heavier boot would be the Sundowner Summit GTX ($200), which has heavier leather than the Wasatch, a taller design, thicker midsoles, and a rubber toe rand to add some toe protection. Id wear that if I was carrying 40 pounds or more and apt to be on the trail for several days. A lighter boot than the Wasatch is the Clarion ($140), a fabric-and-leather boot that draws from dayhiker technology to offer a light boot for fast traveling.
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