The verbiage surrounding the productions here (47-50) doesn't, I find, help very much. Neither do the productions; they are mostly there to help programmers understand the way these things fit together. To learn how to use element content models, look at lots of examples; there are two or three a little later in this section.
One note of real-world caution: although the content-model grammar allows you to build insanely complicated rules, most people don't, most times, thank goodness.
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