A FIS is not only available outside CAS. If I was working an air rally inside CAS, I would probably give a FIS. A RAS is available outside regardless of met conditions (IMC/VMC), however you accepting it means you must fly IFR. The controller is taking responsibility for separation between you and other known a/c, however if you cannot comply because you can only fly VFR then it is not for you.As you are new to this then I'm guessing you only fly VFR , when you have built up more hours and experience, there may be times when you are flying VFR in VMC and if the wx drops, you may then request, if you are suitably qualified a RAS flying IFR. This also means that inside your CAT D airspace, you could fly SVFR or IFR. It can be confusing .You seem to have the gist of it but it is not as black and white as you imply. See if you can access the MATS pt 1 . If you can't , google it, it is available online free.There is plenty about this sort of thing, especially the airspace and ATS services