Hello.
This function allows you to update postings in other components, which are not to consume budget in Funds Management, statistically only in Funds Management. There is no availability control for statistical postings, the budget is not used by the postings.
In other words: statistical means informative 
More info:
Statistical Postings (SAP Library - Funds Management (FI-FM))
Identifying Statistical Postings - Funds Management - SAP Library
Regards,
Oswald