It typically refers to the upper limits of the speed range of a vehicle. Other terms would be "Big End, Up Top, Snort etc ,etc"
My JS 300 will do a screaming 28mph on the top end.
Top end also refers to the engine componets above the cranckase on an internal combustion engine, minus fuel induction and exhaust. Most of the time refering to the condition of the pistons and cylinders.
Dont forget, "top end" is also that thing SJ's cant do lol
My JS300 only has 110 psi compression. I need to freshen up the top end and sell it to an 11 year old:biggrin:.