Calculates historical, i.e. realized of any underlying. If frequency is not the daily, but for example 6h, 30min, weeks or months, it scales the initial setting to be suitable for the different time frame.
Examples with default settings (30 day , 365 days per year):
A) Frequency = Daily:
Returns 30 day , under the assumption that there are 365 trading days in a year.
B) Frequency = 6h:
Still returns 30 day , under the assumption that there are 365 trading days in a year. However, since 6h granularity fits 4 times in 24 hours, it rescales the look back period to rather 30*4 = 120 units to still reflect 30 day .