Hi all :-)

I'm about to write documentation for an already existing system. I.e read source-code and interview developers. The reasons are:

• Detailed documentation for the system is not good enough
• The documentation shall serve as requirement-input when a new system shall be implemented.

The result shall be possible to read by any stakeholder. Yet it shall describe details on a technical level.
I want to write something like this :
"1. The system receives the file and archives it without analyzing the content. The archiving serves auditing and history purposes.
2. The system detects the file’s message type and determines that the file is an XYZ Interchange file containing one or more AAA / BBB /messages.
3. Next, the system filters the file, removing any Carriage Return (CR) and Line Feed (LF) characters that may have been added during the file transfer. CR LF characters are never part of XYZ messages and must be removed.
..."

It looks like usecase but it's not. A usecase is more WHAT shall happen. Not HOW it is done.

Q1: Which artifact suits best to use for this kind of documentation? (Sequence-diagram perhaps? Diagrams might be tricky to understand for everyone though).
Q2: The system have ~100 unique scenarios that shall be described in this form. Most of them are very similar to each other, but with small differences here and then. How can I write so few "how-usecases" as possible and still fetch all details? Refer to a list of special requirement or similar?

/Best Regards
Joakim Carlsson