Some HID's are sensitive to base orientation (Metal halide,multi-vapor,multi-vapor II, and low pressure sodium, SOX), and even when properly oriented Horizontal and vertical burning versions with the same wattage often don't have the same output. Usually the part number of the lamp will clue you in about burning position.
MXR175/VBD is an MXR175, Vertical burn, base down
MXR175/VBU is an MXR175, Vertical Burn, base up,
MXR175/HOR is an MXR175, Horizontal (+/-1 15 degrees) burn.
If you run them in the wrong configuration, they either don't light properly, have low output and/or short life.
Base down is 17,200 lumen, Base up is 16,300, Horizontal is 15,000 for an MXR175 (Metal Halide 175 watt).
HPS (high Pressure Sodium), and True Mercury Vapor don't care much about vertical/horizontal.