Check Key : 

Check key is used to either allow or disallow account assignment entered by the user.

If this indicator is set, the system checks whether a G/L account or an account assignment has been specified on the item screen. If so, the system copies this data to the posting line.

If this indicator is not set, the system always uses the G/L accounts or account assignments determined automatically

Note : Even if the check indicator is set , account determination should be maintained in OBYC. If not maintained  error message M8 147 is issued.