If you need the clients to fill the cell with specific data than you can go
to Data Validation. You will first need to create a list with all the codes
that you will allow the client to use. You can create this list on a seperate
sheet or on the same sheet. Hi9ghlight the cell(s) that you want to edit,
then go to DATA - VALIDATION. In the SETTINGS tab, chose "list", it will ask
you to highlight your list. Once done, the cell will have a dropdown menu
with your list of codes.