At present, only future clients benefit from updates to the default nominal code list.
If I want to amend 100 clients' nominal code lists, each with 50 new codes, it would take forever to update them all manually. Essentially 5,000 manual changes.
A global nominal codes list would mean only 1 set of changes to update all clients at once.
Pretty much all accounting packages (including IRIS accounts production) have options for both user specific and global nominal code list amendments to existing and new clients.