Not sure if you got a reply to this, but basically the question is what is the maximum fluctuation of temperature and relatively humidity over the course of a 24 hour period expressed as a percentage. As an example, say that the temperature in your gallery gets as high as 74 degrees and as low as 70 degrees over the course of a 24 hour period. The average temperature over the 24 hour period is 72 degrees, the fluctuation from the average is 2 degrees. To get the variation percentage you divide the fluctuation (2 degrees) by the average temperature (72 degrees), which gives you 2.7%. The reasoning behind the question is that some materials are more sensitive to large changes in temperature or relatively humidity over a short period of time than they are to gradual changes or even to the actual temperature or percent relative humidity itself. Question 3.16 looks at actual measurements as well as gradual changes (over the course of two seasons).
Hope this helps.