You can 'ride at will' with an STP and there is no need to reserve seats on any trains in Switzerland on the main network because the trains are very frequent and all main trains have plenty of seating capacity.
My quick guide to how railways in Switzerland work follows:
The Swiss railway operating/timetable concept - a quick guide
Switzerland's railways are a lot different to very large countries in many parts of the world, such as USA and Brazil.
Passenger trains run very frequently on a dense network, so that the rail network operates like a whole-country version of the transit of a large metropolis such as New York.
This means that it is not necessary to reserve seats in advance or buy additional tickets except on a very small number of specific special services.
It also means that short connecting times between trains are worked out so that they are easily possible, even with luggage.
The operation of the national network (aka Swiss Travel System) is by what is called the 'taktfahrplan' principle, i.e. that trains run at the same times past each hour at least every 1 hour on most lines, and half hourly where necessary, and all services are programmed to interconnect with each other at nodal points with a minimal connecting time. This minimum connecting time is built in and is designed to enable you to continue a cross country journey without waiting for ages between trains if you have to change trains.
Within this concept, ‘freedom type’ rail passes such as the Swiss Travel Pass and Saver Day Pass act in the same way as 24 hour pass tickets on many city transit systems around the world, in other words they allow ‘hop on hop off ‘ travel on all the main modes of transport which make up the Swiss Travel System – federal railways, private railways, post buses, city buses, city trams, local railways, lake boats.
It doesn't matter which type of train you get on on the main national network (which includes all SBB, BLS and private railways).
If you are worried about short connecting times between key train - don't be! Take a moment to think about how much time it actually takes to change trains at a small and medium sized stations. Short connecting times between trains are worked out so that they are easily possible, even with luggage. Taking 7 minutes as an example. This might not sound a lot of time but think about how far you can actually walk in a straight line in 7 minutes. At an average walking pace even with luggage most people can cover about 700 metres in 7 minutes, which is far further than the distance needed to walk between platform of connecting trains at most Swiss stations (really all except Zurich HB)
Train classifications decode can be found on the electronic version of the Swiss National Timetable, as can timetables for each railway line in Switzerland
Home page: www.fahrplanfelder.ch/en/welcome.html
Explanation of symbols: www.fahrplanfelder.ch/fileadmin/fap_explanations/pdf-Dateien/2022/Zeichenerklaerung.pdf
Train types lexicon: www.fahrplanfelder.ch/fileadmin/fap_explanations/pdf-Dateien/2022/Zugcharakterisierung.pdf
