Time marked with "Billing" is summed into «Billable time» in the case.
 

 
When tick box for «Billing» is marked in work log, time is registered as «Billable Time»:
 

 
Billing is defined on user level and we can in «Admin» tab choose which users to invoice and who will not be invoiced. If user is set to be invoiced, billing tick box will be automatically activated for all   logs and emails   which will be registered in this case.