All contracts are controlled by basic contract law, i.e., there must be a "meeting of the minds" between the parties for a contract to be valid. If the terms of any contract are vague, and it's not clear that the parties really agreed on the same thing, the contract may be held invalid or may be interpreted by a judge or other negotiator. All contracts can be modified -- as long as both parties agree to the modification, and it's not illegal or immoral. And very importantly, never sign a contract until you understand all of the provisions.

Contracts with literary agents are personal services contracts and generally should be revocable by either party under fairly liberal circumstances. They may have a time line or be project specific. (They should be, actually.) Or they may allow for termination with X days' notice by either party. This is important, because a literary agent represents the writer; so the parties need to get along or at least have the same vision. It's also important because if an agent doesn't sell a book after exhausting all of the agent's sources and ideas, the writer should probably look for a new agent.

Periodicals have different contracts than books. They used to be, in the US, first North American serial rights. That was before the computer and internet. Now they often ask for electronic rights. The NWU has a journalism department and CAs who specialize in periodical contracts. Extent of rights and payment terms are particularly critical for those contracts. Who holds the copyright to the article can also be important.

For book contracts, other issues may be more important: What rights are being bought? That information should be specific. Also, if the writer has better contacts than the publisher does for selling some rights (e.g., the writer is a sister of Steven Spielberg), those rights should be reserved by the author. Percentage of royalty and the amount of advance, if any, are important, as are the terms under which the publisher has to account to the writer for income. Liability issues can be very important too: Who has the right to file suit and who pays what, if someone sues the writer and publisher. There are lots of clauses that seem harmless but aren't. An NWU CA can help the writer negotiate through the pitfalls.

Writers for corporations are generally on staff, and their works, at least in the USA, are "works made for hire" under copyright law. Therefore, the corporation owns the works. It is important, though, to note that a work you are hired to do is not always a "work made for hire" under copyright law.