How to automate drop down list allocation given specific entries ?

I will integrate your proposal to our final file.

One last thing maybe- Do you have any idea how we could freeze the entries (meaning convert formulas to text) in Supplier Inv sheet once the allocation has been made. I'm saying this imagining I would eventually change the allocation for usual supplier in "Usual supplier allocation sheet". I would like to keep the history & with VlookUp formula staying in column E & F it would then be modified.

Does this clarify ?

Alex do you mean all the N/A, I can covert the forumula to 0 or blank, then this wouldnt show. This is what I did previously.

I would have to use iferror(vlookup) function. Send me over the final sheet and then I can have a look for you. I have been creating my own database recently. I can assist with this anytime.