To put it simply, the difference between a rule and a hook is how you write it, and hence the power of it.
A rule is merely a number (simplifying here) allowing you to tweak a pre-set behaviour, which a developer has coded. It requires no special skill to use except understanding what the number's impact is.
A hook is a portion of code (let's say 10 lines long) that is called instead or in addition of a regular server computation. It requires you to know how to code a C++ function, but it is much more powerful. In the spirit it is like a short script, just it is in C++, so you need to compile the code yada yada.
Best is to use an example : resist chance for a spell.
Current rules allow you to specify a resist base percentage (ResistChance), a resist multiplier (ResisstMod), a level difference after which no resist is possible for the PC (AutoResistDiff) and the proportion of partial resists (PartialHitchance). The behaviour using these variables is written for you, you cannot change it.
On my test server, instead, I allowed replacing the full resist chance computation with a hook. By defining the hook and implementing in C++ as a function accepting a few parameters and returning the resist chance, I can compute it as I want. In my case I do something on the line of the Shards of Dalaya server, the CHA of the caster influences the resist chance (and the WIS of the target decreases it).
Why not just replace the existing server code ? Because keeping the changes to the original server code is important if you want to integrate new changes done on the main server easily in your custom server. Once the hook is in place (and it can be put in the main server code as by default it changes nothing to the "live-like" server behaviour) custom server developers do not need to touch the original code to significantly tune some rules.
That was my goal. I wanted to implement a score of custom rules, but still keep my server "close to original" EQEmu. In the end I could do most of my changes with hooks and implemented :
- overridding some player choices during character creation
- rewriting the resist chance for spells
- rewriting the melee to-hit chance, damage, mitigation
- rewriting the mana regen
- rewriting the fizzle chance
- rewriting the AC computation (the client cannot show the server-used score, too bad but inevitable)
- rewriting the EXP per level
With minimal and not disrupting changes to the current server code.
I hope it helped explain the difference between rule variables, hooks, and utter server code change. The idea is that hooks could be part of the regular server code "as options ready to be used", on the developer choice, and without having such checks like "if server is guild wars". It would be instead : if designer wants something special we call it, otherwise "live" implementation.
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