> If you set it up to sell when the prices drops to $320, you are guaranteed that it will sell at a price below $320, not that it will sell at $320.

IIRC, that’s closer than the naive interpretation (“you will sell at exactly $320 if the price drops to or below that level”), but it's still not quite right. Unless I'm mistaken, a stop-loss @ $320 is actually “if the price is at or below $320, submit a market sell order”. In theory, it could execute above $320, though at or below is vastly more likely. And, as you note, you aren't guaranteed it will execute at all.

To be precise: As soon as a trade <= 320 is printed, the stop loss turns into a market sell order. It could sell over 320, it could sell under 320 (though much more likely the latter).

There are also stop limits, vs stops which just sell at market. there’s also OCOs and OSOs that can make things interesting.

with a stop limit, you'll simply be left holding the stock if it drops below your limit. depending on what you expect the market to do, this could actually leave you in a worse position (ex, if the value keeps dropping). I'm sure you know this, but, understanding your exit from a speculative position is just as important as your entry.