What is the difference between "Never Invoice" and "Mark Not Billable"?
Never Invoice: "Never Invoice" is for office time for a client you will NEVER BILL THE CLIENT ANYTHING and it will not show in realization report.
Mark 'Not billable': If you EVER bill the client, but in your current situation you are choosing to 'Mark Not Billable" for some reason (for example, you made an error in some work you completed for a client so you are not going to bill them for the time your work in correcting the error) you would select 'Mark Not Billable" on the billing dashboard or change the time entry on the invoicing preview to Include on Invoice = NO so it's on your realization report.
The big difference is that "Never Invoice" is excluded from the realization report.