Let me give you the stupid simple answer first, since if its acceptable, then there is not a reason to hurt your head with complexity. Its where David Hawe was leading to ....
A great unique number wheel is the [Record ID# field]. Look at you current number across all your various departments and find the highest number. let say that is 1,500.
So you could then use excel to import say 2,000 dummy records (or maybe 1,980 if you have already been testing and used up some [record ID#] numbers).
Then we will make a formula field such as
ToText([dept number]) & "-' & right("00000" & ToText([Record ID#]),6)
so that formula will generate the dept number, then an dash and then the record ID# zero padded on the left to be always 6 characters.
The sequence numbers will skip numbers when the Bronze takes say record id 2000 and then a furniture item is created and it will take the next sequence # 2001
So the numbers would look like
10-002000
20-002001
20-002002
10-002003
20-002004
If that is good enough not to actually be sequential within each department code, then that is your KISS solution.
If not, there are more complicated solutions using summary fields, snapshots and lookups. But stupid simple always has its merits.
