All mountain? Overmountain is where it’s at now bro.
Anyhow, AM:
Take a XC racing bike – v light, weight forward on the bars, short travel suspension. Climbs like a rat up a drainpipe, bit of a handful taking it downhill on the rough stuff.
Now take a DH bike – heavy, weight way back, big suspension. Descends like the wrath of God. Doesn’t go uphill.
All-mountain design seeks to marry the two and make an allrounder that can climb just fine yet still be shellacked down the trail. Not too heavy that you couldn’t ride it all day. So you end up with a bike that is either superbly versatile or a crippled compromise, depending on your point of view.
It’s an exercise in marketing, to a degree, because you can ride a XC bike on any trail in the UK justabout. But if you look at a XC bike and an AM bike they’re clearly different and designed around different parameters. So it’s not total bollox, IMO.