What makes a particular paragraph into a heading or something else is the properties you assign to the paragraph. Assigning properties to a paragraph is no different from assigning a style in a word processor, and usually it's just as easy. In a nutshell, you type a line or block of text and then assign properties to that paragraph to identify it as a heading, body text paragraph, or whatever. Voilà.
Composer calls each discrete chunk of text—all the text between paragraph marks (the character you type when you press Enter)—a paragraph, whether it's a heading, one line in a list, a multiline paragraph, or just a bunch of words.
Note that paragraph properties apply only to entire paragraphs. For example, you cannot format two words in the middle of a paragraph as an address and the rest of the paragraph as a heading. Either the whole paragraph is one thing, or the whole paragraph is something else.
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