You can always log on even if your ADVANCE account has no funds.
When you attempt to release a print job the print server will raise an exception that you have no funds.
The FDI will be enabled only if the users balance covers the cost of at least 1 copy of the lowest priced copy line.
The gotcha is that the default price for a copy line is $0.00 so if the administrator has not set all the prices then the FDI will always be enabled even for ADVANCE users with a negative balance.
When the user dials up copies for more than they have funds for the Xerox iMFP does not notify the user.
When the copies use all the funds the FDI is disabled - some device will complete the dialed up job and others will complete the copies in transit.
I don't know the details of how each Xerox behaves.