My GM is completely new to the 4e rules and is convinced that Sustaining foci are limited to just one spell. As in each foci is attuned to one spell, and you can only ever use it for that one spell. Is he right or totally crazy? I've never played an awakened character before this one, but from what I've seen in my other groups, he's wrong >.>

He's wrong. They're specific to a single *category*.

He is...CRAZY.

Hes crazy, think of it as a gun in which you can load different types of ammo.

You can always dismiss the sustained spell or it can be disrupted, in either case you can stick the same one in it or choose another one.

all Spell Foci are limited to one category. Spellcasting, Sustaining and Counterspelling.

So each one is attuned to a specific category of spells, and it can only be used to sustain spells in that category, correct? And how do spellcasting foci work, do they only add dice to the category they're attuned to?

Yes.

Cool, thanks guys.

You can only sustain ONE SPELL AT A TIME, but once that spell is disrupted (Voluntarily on involuntarily) you can cast another spell and use focus to sustain it. It does not have to be same spell or even in same category.

Well, that last point is wrong. While the Spell may be different, it MUST fall within the same category of the Focus.

Illusion Foci only Sustain Illusion Spells
Manipulation Foci only Sustain Manipulation Spells...
And So On...

It's right there in black and white, Bushw4cker.

If it helps, tell him he's remembering 1st/2nd edition spell locks, which were specific to a single spell. In 3rd edition and beyond, they became category specific (and renamed Sustaining Foci).

Your right, can't believe I never saw that 8/

In my defense, it is really easy to miss if you don't read carefully.

I avoid most of the headaches surrounding focus categories by sticking with a power focus and one or more sustaining foci for spells I use a lot; that way I only have to keep track of the categories of the sustaining foci. Much easier (and more resource-efficient, too!)