What’s the difference between symbolism and allegory, as explained by Foster?

From the text:

Here’s the problem with symbols: people expect them to mean something. Not just any something, but one something in particular. Exactly. Maximum. You know what? It doesn’t work like that. Oh, sure, there are some symbols that work straightforwardly: a white flag means, I give up, don’t shoot. Or it means, We come in peace. See? Even in a fairly clear-cut case we can’t pin down a single meaning, although they’re pretty close. So some symbols do have a relatively limited range of meanings, but in general a symbol can’t be reduced to standing for only one thing.

If they can, it’s not symbolism, it’s allegory. Here’s how allegory works: things stand for other things on a one-for-one basis.