What is the difference between Normal calendar and special calendar?

In GL we can define 2 type of calendars. One is Calendar (normal Calendar)and another Fiscal calendar.

Normal calendar uses the year in which an accounting period begin for the system name .
Fiscal Year uses the year in which your fiscal year ends.

In case of special calendars they will defined in AP module as type Recurring invoice payment terms withholding tax key indicator.

Recurring invoice payment terms withholding tax type calendar used for respective transactions as name indicates. Key indicator type calendar used for key indicator report