When looking at the trains from Munich to Salzburg, what does "M" mean on the type of train service? RJX is basically a regional express train right?
"M" is Meridian, the operator of regional trains between Munich and Salzburg. "RJX" is Railjet Express, a long distance train operated by OEBB, the Austrian National Railway. So a Bayern Ticket is valid on "M" trains, but not on "RJX" trains.
Meridian is a private company that operates trains on the Bahn rail system on a Bahn schedule. They recognize Bahn tickets and "passes", like the Bayern-Ticket, but also have their own passes, including the Guten-Tag-Ticket. The Guten-Tag-Ticket costs €2 less than the Bayern-Ticket (€23 for the 1st person, €7 for persons 2 - 5) but is valid only on Meridian trains. Unlike the Bayern-Ticket it cannot be used for local transport in Munich.
Even though it costs €2 less, the Guten-Tag-Ticket will not be shown as a regional pass option for Munich Hbf to Salzburg on the Bahn website.
Meridian also operates, amongst other lines, the Bayerische Oberland Bahn (BOB) running south in three branches from Holzkirchen to Tegernsee, Leggries, and Bayrischzell. The Guten-Tag-Ticket is also valid on the BOB.
The Railjet is an Austrian train and can only carry passengers from Austria to one stop in Germany, the Munich Hbf. It does not stop at Munich's Ostbahnhof, which means it is not as handy for travel from MUC (airport) to Salzburg, a popular route.