Note that if you have enabled the Allow Creation of and Show Fiscal Calendar Options  (unanet.fiscal_calendar.enabled) property, additional 'fiscal' period options will be available in the drop-down.  
Fiscal periods must be established via the Admin >> Setup >> Fiscal Year screen in order to be available for use in reporting.  Specific Fiscal Periods appearing in drop-down lists will be a concatenation of Fiscal Year Name w/ Accounting Period.