Posting period variant controls weather any accounting document can be posted in the period or not.
Document can be posted only in period which is open for posting.
To restrict document posting in future months and previous months, corresponding posting periods are closed.
Posting period variant allows maintaining period control for
Specific account type
Example: Document posting in a period is allowed for customer but not for vendor
Specific account number
Example: Document posting in a period is allowed for few customers but not for others
Example: Particular users are allowed to post document in the period but not others.
posting period variant
Standard practice: Normally only one posting period is kept open.
Document posting is allowed only in current month hence only one posting period is kept open.
But when moving to the next month, certain month end activities need to be performed. Documents need to be posted in previous month hence both posting periods new and old are kept open for few days.
Only month end activities are complete, old period is closed and only one period (current month) is kept open.
Link to read more about posting period variant http://help.sap.com/saphelp_erp60_sp/helpdata/en/96/8b300943ce11d189ee0000e81ddfac/content.htm