Is it possible to rename the column headings?
My company do not use VAT, is it possible to rename the VAT column to something else such as Client Ref?
You can’t re-purpose existing columns to hold different types of data. I believe @PeterK already replied to a separate post stating that the columns were fixed and non-customisable.
The best you can do is rename the column headers on the actual invoice to read something else. This can be done using CSS transforms as explained here, a warning in advance it is a more technical part of the system.
Either that or do what quite a few other users do and put any specifics regarding the order into the notes section.