There's a lot of info in the Realms of Magic chapter (about the 4 realms, why they happen, what critters are specific to each realm; about regiones, & where the vis is, etc.) in the Ars Magica corebook - I was wondering how much do the actual mages know, in general (some houses or specific mages will know more, I guess)?
Should I just assume that this is "public" info for all mages, or should only rolls by specific knowledge skills (like "Knowldege: Fairy Realm") bring forth the info?
In my sagas, generally speaking, all magi know about the four realms, what sorts of creatures are associated with them, where auras tend to occur and the basic mechanics of regios. I generally only require Realm Lore tests for detailed or specific information.
My take is that all of the basic information present in the core book is widely understood across the Order. Content from the four Realms of Power books is probably more advanced and you can imagine largely known only by specialists.
The important thing is how player characters relate the current story events back to their knowledge of the realms and that's where the Lore scores/rolls come in. They allow the storyguide to help clue the players in as to the important factors in a given scene or story.
I base a General Knowledge Ability for "Magic Stuff" (not one that you spend xp on or need a score in) on House , Magic Theory Score and Art/s Score.
A Magical Focus of some sort indicates specialized knowledge in the area covered.
These also help decide Ease Factor rolls for Magic Lore in an area that a player character knows about.
If the player has no score in Realm Lore of any kind , he can still roll to know about things he has studied that relate to it.
Basic knowledge of Order of Hermes stuff (no score or zero) will vary with House.
I'll write stuff like that up for games I run. (Even non Ars stuff) For me it's a holdover from running vampire LARPS I think. For Ars Magica I think it helps encourage Magi to actually put points int the lores. Write up an outline for every lore. Cut and paste the levels onto a sheet and pass it out to the players. Also during an adventure you can have cluse ready to pass out that are available to those with the right ability.
If I remember the last time I wrote some up for ArM I put knowing the general mechanics of auras, might, and regiones under Hermetic Theory. Lore gave you more general knowlege about where to look for things and how they come about.
One thing that tends to be overlooked (ims , anyway) for "What Magi Know" , is libraries.
If you have Magic Lore 0 and roll a stress die of say (1,1,1,9) ,
then you probably spent a large chunk of your apprenticeship copying the relevant books on the topic.
As your Covenant may not have all the reference books you need , your student notes refer to the tomes you studied.
A trip to Durenmar may be required.
What all magi know from gauntlet is certainly something that it would help to define. I am slowly writing this up for my saga. I want the players to know that there are things like Sorcery, Theurgy, Thaumaturgy etcetera so one might start looking into one. This might come from MT, Magic Lore or even OoH Lore.
I do think it is important to distinguish between (Hermetic) Magic Theory and Magic Lore. The latter is good for those thinking outside the Order.