Still being new to Shadowrun, some of the simpler things take a while for me to figure out

Does the spirit's category (combat, illusion, detection, etc) matter for everything I ask it to do? or just if I'm using the aid sorcery/spell sustaining type power? IE, can I send my Illusion category spirit into combat?

I had assumed I could only use an Illusion spirit for Illusion-type tasks, but after rereading the rules I'm not sure.

I'm not a magic expert, but I've never seen such an arbitrary restriction placed on spirit use. In fact I've never really understood why tradition's spirits have been classified as such. I just assumed it was a suggestion of what those spirits are best at.

Unfortunately, my whole group (including GM) is learning Shadowrun from the beginning. As the sole magician, I get to learn all the intricate spell/spirit rules. Always glad to find out my interpretation of RAW is wrong, especially when it helps me

The Category is only important for the Aid Sorcery / Spell Sustaining service.

What Sma said.
Basically it's only important wether your tradition has a specific type of spirit at all, for regular use it doesn't matter which category the spirit relates to. That's mostly for fun and fluff (and the bound spirit services which explicitly mention that restriction).

Wow... this just opens up a whole new world thanks!

Actually, as a GM who's been playing around with ways to give more character to spirits, the idea of using category as a sort of rough roleplaying guide for spirits of that tradition is kinda cool.

Comparing two traditions here, let's take Christian Theurgy and Buddhism. CT classes fire spirits as combat, so a Christian Theurge's fire spirit is a mighty angel with wings of fire, wielding a flaming sword; we're talking the personification of God's wrath, divine justice in holy form. This thing appears and starts smiting shit. If it picks up optional powers the first will most likely be Fear, and the next power it gets will probably be Guard (since Noxious Breath doesn't really fit the flaming sword look).
The Buddhist's fire spirit, on the other hand, is associated with illusion, so we're talking about something very different here. When we throw in the flavour of Buddhism itself, I picture something almost immaterial; a shimmering heat haze, brief flickers of flame, the sense of something indefinable. Like fire it is hard to grasp, hard to control. It's personality is... shifting at best. Much in the style of Buddhist thinking it is a riddle, an enigma. I like the idea that, in keeping with the theme of trickery, it picks up Guard first, thus confounding other people's attempts at trickery (ie, Accident) with it's own enigmatic ways. In fact it would be really cool to add some colour here by describing how the unfortunate event appears to happen, except that it didn't... rocks fall, but somehow the protected character is standing there unscathed, with absolutely no idea how they weren't hurt.
Another comparison would be the Zoroastrarians with their detection fire spirits; this is fire as the gateway to another world, reaching beyond human perception. Perhaps not even flames; a cloud of roiling smoke, fire flickering within. A calmer spirit, with less tendancy to violence, it is more of a wise counsellor. It's first optional power will obviously be Search, and it's second will probably be Fear or Noxious Breath (in keeping with the smoke idea, if you go that way with the look).

I mean the rulebook makes much of the idea that the spirit classifications are just stat-sets, and that the flavour should depend on the tradition. I think using the category as a rough guide to roleplaying the spirit is a cool way of making those differences more obvious. I mean it actually gives us demonstrable differences between traditions like Hermeticism and Chaos Magic. Just by reorganising Man, Water, and Earth into difference categories we have changed the dynamic of how each tradition works. To the Hermetic a spirit of Earth is associated with manipulation; it represents plasticity, the morphic, workable nature of materials such as clay and steel. To the Chaos Mage Earth, associated with health, is the vital, primal forces that make up the Earth itself; it is the Gaia force that has been worshipped under different names by thousands of different cultures throughout the history of the world. Hermetic mages see spirits of Man as a representation of the human essence, the one element they cannot define as part of the material world (Earth/Air/Fire/Water), so they associate it with health. After all, lose too much essence and you die, and even losing some makes you harder to heal. Chaos Mages, on the other hand, call up spirits of Man as the trickster spirits that have appeared in almost every culture, deceptive and guileful manipulators of mind and perception.

The more I think about this, the more awesome it is, and I'm annoyed at myself for not spotting it sooner (cause it's kinda bloody obvious really).

I definitely see the tradition-specific sides of the spirits. And certainly, as a Norse mage, my troll is going to choose to go into battle with Valkyrie most of the time. But it's good to know that isn't a set-in-stone requirement.

I especially like Earth spirits as Manipulation... seems so perfect for their Binding power to manifest as mud and vines catching hold of someone's legs and sucking them in.

Meh. It's another case of depending on the player to provide depth, instead of the material providing depth.

But back on topic, the category of spirit connection doesn't define what services it can do for you. What does matter is what the spirit is capable of. A spirit of man is mediocre at combat, while a spirit of Fire has lots of combat-related powers. On the other hand, a spirit of man has many other useful powers, which allow it to perform other tasks for you: ones that don't require brute force.

You certainly can send your "Illusion spirit" into battle, although "Illusion spirit" is not the best way to think of it.

Look at it this way. As a mage of Tradition X, you can summon 5 types of spirits. They can fight for you, they can use their powers (Accident, Guard, Search, etc.) on your behalf, they can do all sorts of things for you. They are limited only by their nature (Spirit of Man, Spirit of Fire, etc.)

The only exceptions are Aid Sorcery, Aid Study, Spell Sustaining and Spell Binding. Only in those four cases does the type of spirit have to match the type of spell.

And that is exactly where my confusion lay. Good to have it cleared up. Good to know I can send my Spirits of Fire into battle, even though Norse tradition has them as "Health".