Working with marks

Hi, is there a howto on using marks?

I have a few questions. I want to pass some audio to sox:

ExecCommand1 = "play %f trim %S =%E"

I will do more with it, but for now I just want to test getting something to play.

How do I clear one or both marks?
Does the order of the marks matter? I.E. does the ' need to be before the " in the audio?

I made a " before making the ', and rewound and tried to make a ' but it isn't working.

TIA