These are two different functions.
F12 and its variants in the toolbar menu are intended for list numbering. When you go through More Numberings, you open the same dialog as Format>Bullets & Numbering. This dialog offers 6 tabs. The first four exhibits common list “styles” and preset parameters in Position and Customize tabs when one is selected. If you then tune the settings to your liking, never reselects anything in the first tabs otherwise all your tuning will be wiped away (to revert to default settings for the selection) and you’ll have to start over again.
As said, this is for list numbering and has nothing to do with chapter numbering.
Tools>Chapter Numbering is specifically dedicated to the task of numbering the paragraphs styles with the Heading n family of paragraph styles. The members of this family are also marked so that they are automatically collected to build the TOC.
Unfortunately (for newbies, because very sophisticated schemes are possible if you slightly break the rules, but you better know what you’re doing), chapter numbering is not protected against overspecification: if you apply F12 to a chapter heading already numbered with Tools>Chapter Numbering, you override the chapter number with a list number. The list sequence counter is totally independent from the chapter sequence number. You end up with an unmanageable mess where your headings will bear the list number while the entries in the TOC will bear the (not-synchronised) chapter number. Both numbers will be incremented in unpredictable ways.
To sum up: never, never, never mix list numbering and chapter numbering.
Chapter numbering is reserved for chapter numbering (strange, don’t you think?) and this is complemented with special properties for Heading n (this family is internally managed to behave a bit more smartly than other styles) and with the TOC collection engine.
I watched the YouTube video. There is a big flaw in this “tutorial”. It works because the author doesn’t need any numbered list in his document. But should he, he’d be faced with a very big issue: either he messes up his heading numbering with “holes” corresponding to the list items and his lists will be strangely numbered (in sequence with headings). If he fixes the list numbering through resetting it, he’ll simultaneously jeopardize his chapter numbering.
Among other flaws, his text is Default Paragraph Style instead of Text Body. This wouldn’t have occurred if he had styled his headings Heading n while typing it (very easy: Ctrl+“n” with n=1-5). You automatically swith to Text Body when you hit Enter.
Also, View>Formatting Marks and View>Text Boundaries are not activated. These options display (non-printable) gray lines and blue symbols showing you what your document really contains. This is very convenient for an author to make the difference between empty paragraphs and empty vertical space.
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