You have the possibility to customize what fields will be mandatory when adding a new fuel receipt.
To do this:
Go to Fuel ➡ Fuel receipts
Click the Settings icon on the top right.
Go to the "Receipts mandatory fields" tab
Mark the checkboxes of the fields you want to make mandatory while adding receipts
The greyed out fields are always mandatory for receipt registration.
"Refuelling source" is only mandatory for the "Refill from unit" form.
"Charging start time" and "Charging end time" are only mandatory for the "Charging receipt" form.
Click the SAVE button at the bottom.