The mandatory fields are un-checkable (cannot hide). Using Purchase order as an example, fields like Item, amount, tax code, tax rate and gross amounts are mandatory, and we cannot simply uncheck them.
Form edit mode: mandatory fields are dim and checkboxes cannot be unchecked.
To hide the field on a particular transaction form, i.e. Amount, you can just empty the label of the field and save the form.
Once the field label is removed, the field will no longer display on the transaction form, and it will not be printed as well.