I've never seen an endmill that was meant for a secondary support.. Not to say it doesn't exist but I'd think it likely isn't meant for that or there wouldn't be flutes to the end. Horizontal machines use arbors because you can make up essentially any profile you want in a near endless size (obviously there are limits to the overarm and table and arbor), and cutters can be reground for them a lot easier than endmills. I'd guess it's just a run of the mill extended endmill, it wouldn't be meant for roughin, just cleaning up sidewalls all the way down.
I've also seen very long endmills used in timber processing machines, a place I worked when I was 16 ended up buying a humdinger k2 that could handle 24"x18"x30'+ beams, it would use endmills like that for pocketing for the tenons. They will use carbide in wood processing as well, but it's not unheard of to use hss for that.
Ps, autocorrect can suck my balls.. If there's mistakes in that posts spelling or entire words you will have to read between the lines.