Hi Bernard,
It all depends on a miriad of factors. They are different if you have reasons to make them different.
I've generally seen all requirements document in the same Business Requirements Document (BRD). However - at times there are business reasons why there are different. In the case when the business requirement comem from the line of business and need to be baselined - it is a good idea to have those as a separate document. In some organizations, functional requirements tend to be document by the technical design team (systems analysts) perhaps using use cases and those tend to lend themselves well to be in separate documents.
If you have a small team, working on small projects, a combined document may work.
One of the advantages of having multiple documents is the ability of different team members working on different documents/areas at the same time.
Before you create the template(s), you need to gather the requirements for "why" you need this template. Of course - you're in trouble because you don't have a template to capture your requirements. Oops!
Seriously - you need to understand why you're doing this and how these artifacts will be used. Are there any requirements rearding traceability in which case you might want to use a spreadsheet to track requirements or, better yet, an off the shelf requirements management tool. If the goal is to have requirements be signed-off by the business - is there a need to present them in a form other than text (such as UI Prototypes)?
etc. You need to apply business analysis techniques in order to arrive at the answer yourself since there is no off-the-shelf one-size-fits-all solution.
Hope this makes sense!
- Adrian