When you use the [Fa], [Fa1] or [Fa11] holidays tokens, Q++ searches through all the sets of the selected Holidays List to find holidays occurrences. There are some case, however, where you want to use the convenience of having many sets in a Holidays List, but wish to only look for holidays occurrences in the Nth set of that list.
The Specific Holidays Sets Modifier is appended to any of the 3 tokens above in the following form :
[fa:XX] where XX is the position of the set in the a-Holidays
Note that if you want to also include Duration Modifiers, then these should be added after the specific holidays set modifier. For example :
[1fa:03:y+00]
Specific holidays set modifiers are often used to list holidays of many holidays sets over a year in holidays page.

See also : GetHolSetNameOfId.