With a single SDF file, you would define all physical servers that every QMF application user would need to access in the one file. All users would share the SDF file over a file sharing network. A single SDF file is advantageous because it minimizes and centralizes administrative tasks.
For multiple SDFs, you would define a select set of physical servers in several different SDFs. Users would access one or more SDFs depending on their requirements. Multiple SDFs increase your administrative tasks since you must make all changes to the SDFs using the QMF for Windows Administrator application.