Intrinsically there no such thing as a heading in LO Writer. What I mean is “heading” makes sense only for you as a writer, where you usually think of a “heading” as a very short paragraph delimiting some break in the discourse.
In LO Writer, some paragraphs are specifically marked up to optionally receive a generated number and also to be entered into the TOC. This is done with the Heading x paragraph style family. Consequently, you can mark up text paragraphs also with Heading x.
However, “heading” semantics is usually associated with some outstanding formatting, such as larger font size, bold or small capitals, and I assume that you don’t desire such formatting for your text paragraph. Unfortunately, it is not possible to have two series of “heading” styles intermixed in your expected way (albeit it is possible to have several series provided they are consecutive, like NUMERIC chapter numbering followed by alphabetical annex numbering).
The work around is to superimpose a character style over the paragraph: define such a character style to have the same font attributes (size, weight, slant) as Text Body. Select the whole to-be-numbered paragraph, apply the character style. Then re-apply the ad-hoc character styles where you want emphasis or other stylistic variations.
Now a problem remains in case you don’t want your not-“heading” numbered paragraphs to be entered in the TOC. I have no obvious solution for this case.
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