BenQ PB8260 - Manual (Page 38)

Operation 34
High Altitude
A mode for extreme environments like high altitude and high
temperature. We recommend you use the High Altitude mode when your
environment is higher than 3000 feet above sea level, or is hotter than
40℃.
Operation under "High Altitude Mode" may cause a higher decibel
operating noise level because of increased fan speed necessary to improve
overall system cooling and performance.
If you use this projector under other extreme environments excluding the
above, it may display auto shut-down symptoms, which is designed to
protect your BenQ projector from over-heating. In cases like this, you
should switch to High Altitude mode to solve these symptoms. However,
this is not to state that this projector can operate under any and all harsh
or extreme environments.