The categories could be extended to include other marks of puncuation. Keep in mind that spaces are also punctuation. Also, the various marks convey different meanings. When you change marks, you change meaning. Modification often changes the number of items we perceive the writer to be talking about.
All commas are devices for preventing modification.
They work within sentences. S PP the prep. The coordinator is sometimes used with a comma to distinguish it from situations which are ordinary word or phrase coordination. The comma and coordinator should be seen as a unit, a special signal, when used this way. This can happen on the sentence level or within phrases.