A H.A.L.O - high altitude low opening - jump will generally take place between 15,000 and 35,000ft. Due to the low levels of oxygen in the atmosphere above 15000ft, a HALO wingsuiter will be required to wear an oxygen mask. On top of this, the low temperatures at this altitude will require a jumper to wear equipment capable of protecting them from the cold.
The highest HALO wingsuit jump performed to date was over 37,000ft. Generally preparing for such a high exit and acquiring a suitable aircraft is an expensive process and requires a considerable amount of preparation.
Although a HALO jump will often refer to a low opening, a parachute will usually be opened around 3000ft, similar to any skydive.