The Raw Sample Method returns unprocessed raw data from the Historian.
If Raw is the selected Sample Method in an Explorer Trend, the time setting becomes Duration and the duration is selected as 8 hours. The limit of 8 hours, which can be updated in Explorer's web.config file (MaximumRawDuration), is there to restrict the amount of raw data points required for this method.
An alternative method to use for a longer period is Adaptive Raw.