90.1 2010 User's Manual versus the Standard

Intereseting. I always understood it to be a 30 minute max vacancy, even though in practice I've seen it around 15 minutes or thereabouts. In the 90.1 2016 User Manual, the langugae was clarified a bit and reads "... the lighting power of a space must be automatically reduced to less than 50% of the full power within 20 minutes of the last occupant leaving the space." Standard 90.1 2016 reads "... The general lighting power in the space shall be automatically reduced by at least 50% within 20 minutes of all  occupants leaving the space." Ignoring the change in reduction amount and vacancy duration, the language of the standard and user manual align better in 2016. 

My guess is that the users manual in this case is clarifying very confusing language in the standard. What is written in the standard actually does not make any sense. You could interpret that as 10 seconds which would be ridiculous. The users manual makes more sense to me and I would go by that instead of the standard in this case. As Waleed points out you can often look to more recent versions of the standard to gain clarification and in this case it is pretty clear what is intended relative to these controls.