Credit Review Calculation (Open Authorization Value?)

Summary

What type of orders go into Open Authorisation? How is this calculated?

Content

Hi,

Below is the Available Credit calculation

profile credit limit (customer profile) - open authorization (from AR_CUST_OPEN_AUTH_SUMMARY ) - customer balance (from AR_TRX_BAL_SUMMARY)

We understand how the following values are created:

1) Profile Credit Limit 
2) Customer Balance

However, we don't know what is included in Open Authorization? (which is CURRENT_OPEN_AUTH_AMT field from AR_CUST_OPEN_AUTH_SUMMARY table)

Does any one know what makes up the Open Authorization value from a Functional perspective i.e. what type of Orders would go into this value?