Default: Up to interval time
This setting only applies to Bulb Ramping mode. Normally Promote Control will allow Bulb Ramping exposures to be almost as long as the Bulb Ramping interval that they have to fit. With Advanced Bulb Ramping, Promote Control will ramp up the camera ISO, trying to fit all exposure durations within the preset interval. However, if you need all exposures to be considerably shorter than the frame interval, you can set this limit using "Limit bulb ramping longest exposure" setting.
Example: with Bulb Ramping interval of 10", Promote Control Bulb Ramping will allow exposures of up to approximately 8 seconds. However, this setting may lead to significant motion blur. Using this setting, you may limit the longest exposure in Bulb Ramping, to 2" to keep the image motion blur reasonable.
The bulb ramping interval limitation will still apply. Example: with Bulb Ramping interval of 5", and this setting set to 10", Promote Control logic will hit the interval first, and the limitation set by "Limit bulb ramping longest exposure" setting will not apply.
Note that this limitation will require Promote Control to match shorter exposure times with higher ISO ramps. Therefore, when using this setting, consider setting Highest Auto ISO Setup setting to a higher value (possibly up to your camera maximum ISO value). Please see Advanced Bulb Ramping for more details on how automated ISO ramping works.
NB: When "Bulb ramping interval measured as" is set to "Exp end - exp start", default value of this setting will read as "No limit" instead. This is because with the aforementioned setting, interval is not participating in the process of determining the longest available exposure.