In SR4 and its major supplements Arsenal, Augmentation, Street Magic, Unwired and the like, players and gamemasters will run across a lot of Optional Rules and pages with suggestions for Tweaking the Rules. While fairly self-explanatory, these changes and additions to the ruleset can have a serious impact on the game, both for players and gamemasters, and should be considered before being used.
What is Tweaking the Rules?
As shown on p.69 SR4 and p.31 in Street Magic, these are pages with suggestions for possible changes to the fundamental rules in SR4. Some of these tweaks arise when different mechanics are suggested to handle an aspects of the game - the routes not taken, so to speak - while others reflect simplifications, changes to reflect rules in previous editions, or simply popular house rules. The nice thing about Tweaking the Rules is that they often require no additional changes to character stats as written - qualities, skill ratings and the like stay relevant, it is only how they are used that changes. Some of these tweaks are exclusive, but there's nothing to stop a GM from implementing as many tweaks in their game as they can stand.
Hank prefers more specialized talismongers in his game, so he decides to make Enchanting a Skill Group broken down into Artificing, Alchemy, and Vessel Preparation, as outlined on p.31, Street Magic.
Tweaking the Rules is a decision made by the gamemaster and, hopefully, the rest of the gamers in the group - as fundamental changes to how the game works, this prevents players from being blindsided. Ideally, any rules tweaks should be implemented from the beginning of a campaign, or at worse as soon as the new material the tweak would effect comes into play. Since this is not an ideal world, however, many times a gamemaster and players will be exposed to a tweakable rule for a couple times first and then decide that "This sucks!" and implement the rule tweak. That's fine, as long as the change is perfectly clear to everyone when implemented - no one wants to suddenly discover that their character is screwed because the laws of the universe changed on them.
After playing an aspected Enchanter for a few weeks, Rachel and her group are frustrated by her inability to do anything outside of Enchanting. After taking a vote at the table, the group asks Hank if they can apply the Expert Aspected Magician tweak, and Hank agrees.
What are Optional Rules?
These are rules that are essentially designed to be used with the system as written, but which are optional because they have the potential to complicate play - which is fine when players and gamemasters are looking to complicate their lives a little, but which can slow down a game and generally be a nuisance otherwise.
Normally Hank doesn't spend a lot of gameplay on Enchanting, as it slows down the action, but now he wants to run a storyline highlighting Rachel's Enchanter. To make it more interesting, Hank decides to use the Exotic Reagent Requisite option on p.83, Street Magic.
Technically speaking, Optional Rules fall under the yea or nay of the gamemaster, who has the right to allow or disallow them as they see fit, but they're really there as character options as much as anything else. It is encouraged that both players and gamemasters use Optional Rules when they want to do so. Unlike Tweaking the Rules, Optional Rules are easier to implement during play, as they represent additional options and expansions on the ruleset rather than fundamental changes to the rules themselves. This doesn't mean they can't be unbalancing, however, and gamemasters should be aware of the potential for abuse; players for their part should always ask the gamemaster before using an Optional Rule - it's not just polite, it gives the gamemaster time to prepare.
Faced with a shopping list of difficult-to-find reagents, Rachel decides her Enchanter needs a little help. She asks Hank if it's okay for her character to use the Aid Enchanting rule on p.79 of Street Magic - as an Expert Enchanter instead of the regular aspected magician type, Rachel's character has no trouble with a little Summoning and Binding, even though she isn't great at it. After some deliberation, Hank rules against it, on the grounds that it would make the adventure too easy.
Cuts Both Ways
The bottom line with Shadowrun is to have fun, and few people have fun playing a game where the NPCs are given advantages over the PCs, especially when the players can sense those advantages but don't know what they are. This basically amounts to fair play on behalf of the gamemaster: if you are Tweaking the Rules or using an Optional Rule with regards to an NPC, it is only fair to extend the same rule to the PCs.
Later on during the game, the PCs encounter an NPC Enchanter using a bound spirit to Aid Enchanting - Rachel upset at this because she was just told her own character couldn't do the same thing. Hank initially tries to cover his ass with an in-game explanation, but after seeing how upset Rachel (and the rest of the group) is, he decides that what's good for the goose is good for the gander: Rachel's enchanter (and any other PCs) can use Aid Enchanting as well.
Some gamemasters and groups can take this the other way and let the players decide which Optional Rules and Tweaking the Rules they want to implement, with the caveat that any tweak or option they agree on becomes fair play for the gamemaster-controlled NPCs as well.
Rachel's Enchanter is a mambo, and she decides to craft some corps cadavres using the option on p.95, Street Magic. Hank allows this, but with the caveat that now the PCs might face some NPC-crafted corps cadavres as well.
Why Not Just House-Rule It?
The two big advantages to using Tweaking the Rule and Optional Rules are balance and compatibility. From a strict balance standpoint, all of the tweaks and options were written by the game designers and subject to playtesting - they can be expected to be about as balanced and easy to fit into the game seamlessly as can be expected. Compatibility is more of a metagame issue; other gamers and gaming groups are aware of the tweaks and options printed in the various sourcebooks and may use them themselves - at the very least, they have a common reference. This is nice because you can take your character sheet from one table to another, or even to a convention, and it will almost certainly be acceptable - even if two groups or gamemasters use different tweaks and options, they can make a ruling there and then without having to go over house-rules.
Rachel decides to take her enchanter character to a game in Goblyn's Market, a local con, where a few sessions of Shadowrun are being played. The GM notes that Rachel has Artificing, Alchemy, and Vessel Preparation as skills instead of Enchanting, with specializations in each, and that her character sheet has two corps cadavres listed. While the GM doesn't use the Enchanting tweak himself, he's familiar with it, and doesn't dismiss Rachel's character out of hand, but tells her that since this game isn't played with that rule, he'll only count her lowest Enchanting group skill as her Enchanting skill rating for this adventure, with one of the chosen specializations. The corps cadavres he has no problems with, as the GM uses that optional rule anyway.