It means you may be using different process units more often or less often within the refinery. More of your feedstock might be going through a certain process unit at a particular time because of seasonality issues than might be going through that process unit at some other time of the year. There are process units and there is impact on feedstocks to be considered.
There are significant operational changes within the refinery to meet the seasonality requirements of both gasoline and diesel. They can be quite significant. As well, there are multiple changes throughout the year, depending upon the geographic region and the prevailing climatic conditions. Largely it's ultimately climatic conditions, seasonal differences in climate, and seasonal differences in weather that cause significant changes to the recipe, if you will, for gasoline and diesel, depending on those seasonal changes. It involves different uses of different process units and using them more or using them less at any given time in the year.