Quite a while ago the Implementors decided to limit people in their use of craft and the other skills. Allowing players to restring items to look however they want is a pretty big, and quite nice freedom...however there is great potential for abuse. In that regards, here's an idea of how the restriction works.
When making an item, it shouldn't contain materials that weren't used in its creation. For example: when forging a sword, you use metal stock, you aren't using gold and pearls. So you should not include such items in its description. This really is a no-brainer, but people don't seem to get it.
When crafting an item (the process of changing its description, it uses the skill 'craft'), things are a little more complicated. Players are restricted to two general types of materials that they can work with. The accepted list includes: metal, leather, cloth, jewelry, wood, parchment. Other materials can be added if need be, but nobody has really requested anything else.
This means that if someone comes to you (say a metal/leather person), and asks you to change their shirt, you can't do so and you send them to someone who can.
All players with the crafting skills are required to adhere to these guidelines, whether a member of the guild or not. Failure to do so will greatly anger the Immortals, and will likely result in your lose the skills involved, or at least having them set down significantly. The guild maintains a list of who works with what materials. If you are not on this list, you better get on that list as soon as you can. Just contact a Craftsman, or any Immortal.
As a side note, players may be interested to know that (given a bit of permission from the Immortals), they can learn to work with more materials than just the 2, but it would require extensive training from someone who already works with those new materials.