This setting is only used for Bulb HDR or Bulb Ramping modes. This setting controls how much of a safety margin Promote Control will apply when doing Bulb Ramping and Bulb HDR on exposures close to the shutter closing limit. All cameras have different shutters, and some higher end cameras (such as Canon 5D Mark III) may have shutter close times that are more consistent than others. If your camera is a higher end type, you may try changing this setting to "Minimal". This may offer a shorter Bulb Ramping / Bulb HDR exposure that can be obtained without having to resort to ISO ramping / ND filters. However on cameras with less consistent shutters, "Minimal" setting may lead to less consistent exposures. If in doubt, leave at default.