If I cancel a United flight, from what I understand I'll get credit toward a future flight, but I'll have to pay a fee of $300. But what I can't seem to find out is

- Does the future flight have to be the same departure/arrival airports?

- Does this credit expire (e.g., after  a year?)

- Can I apply credit toward any future flight (perhaps with the restriction in the first bullet), or do they restrict to a small subset of future flights?

Maybe there's a page somewhere that explains all this, but I can't find it after searching both the United website and Google.