Re: SharePoint Document Library Formula Sequential Numbering unique to each prefix
Just confirming - is this for a List or Library?
If it's for a List, then can give you a fairly nice solution as you MUST enter all required fields before you can save the item.
If it's for a Library, then it gets more complicated as you can upload a file and the metadata (including required fields) won't be added initially. So, if you're trigger is When a file is created, it likely won't have the Department or Document Type values to build the unique id.