Hi lally,
1. If you select Calendar, accruals are made according to the calendar year and if you select fiscal, accruals are made according to the fiscal year that is defined in the Fiscal calendars form for the fiscal calendar that is selected in the Ledger form.
2. f you select Calendar in the Calendar type field, you need to select a posting frequency but in case of Fiscal in the Calendar type field, the Posting frequency field is not available. The posting frequency then is equal to the number of periods in the fiscal year, as defined in the Fiscal calendars form for the fiscal calendar that is selected in the Ledger form will work.
Refer below link for more details -
technet.microsoft.com/.../aa570123.aspx
Regards
Sanjay Gupta