What is high-velocity furnace filter?
High velocity filters as it applies to Trane/American Standard filters are the black plastic filters that come with the furnace.  They are designed to have very little resistance and can fit in the bottom of the furnace.  The problem is they don"t filter that much.  The common problem that virtually all modern furnaces have is that they make the furnaces so small that they can not fit a proper size filter in the furnace itself.  A 3 ton air conditioner for instance, requires 1200 cubic feet of air per minute to operate properly.  Assuming a relatively good MERV 10 filter, the filter would need to be at least 20" X 30" X 1" or 14" X 25" X 2".  Those are not going to fit in many furnaces.  The installer would have to install an additional filter rack to fit these filters (which I would recommend)if you want proper air flow and good filtration.  In order to have as much air flow in the limited space available the manufacturers specify high velocity filters that don't filter very well but it is better than starving the furnace for air.  A dirty high velocity filter will still flow more air than a brand new high MERV filter.