What's the starting envelope of APU?
The starting envelope determines the range of altitude (and possibly temperature) at which the APU can be started in flight. In your case, the manufacturer specifies that the APU can be started at altitudes between 1000 ft below MSL and 39,000 ft above MSL, which is also the operating envelope of the APU.
Here is an APU operation and restart envelope of an Airbus A320:

(source: Airbus A320 Limitations - APU)
The cyan range is the operation and restart envelope (as a function of altitude and temperature). This APU model can only be used for bleed air and electrical power up to 22,500 ft altitude. Above, it can only provide electrical power. It also says that the battery is capable of starting the APU below 25,000 ft.