Soutenu ~= quick, fast (in this context)
Calme ~= slow
This option appears to apply to exams that require multiple sessions.
Soutenu: the sessions will be chained in short time (say one or more sessions per consecutive day)
Calme: session programing will be somewhat relaxed.
In the case of FCL 055 the exam requires just one day so you have to select "soutenu" and 1 day.
PS. "rythme" means pace, tempo, cadence.
Hope this helps
DJ