The pre-defined ol.Decimal style name is composed of two parts: the Tag and the Class. The Tag is the html element you want to style. The Class is the style name. You know how in Word there are Paragraph and Character styles? The tag part is similar to these categories in Word. The Class part is similar to the style name you would select (Normal, Heading 1, Caption.) It's not exactly the same, but it's a useful comparison to keep in mind, at least when starting out.
So ol.Decimal is saying "I have a customised ordered list style, and I've called in Decimal". Keep in mind the Class part is just a text string and isn't linked to how the style is set up. For example, you could call a style ol.Turtle. All you know from this name is that it relates to ordered lists. You need to go in to the style and check the Style field for what numbering scheme is used. Again, this is the same as Word, where I could call a style Rumpelstiltskin and all I would be able to tell is that it was either a paragraph or character style and I'd have to go in to the dialog boxes to find out about the other settings.
When setting up a style you can start fresh by clicking creating a new style and selecting the Tag and typing a new Class name. Or you can duplicate an existing style and rename the Class so it's unique for the Tag. In both cases, make all your changes in the new style you created. Levels, font and size, numbering scheme, etc.
As I mentioned in a previous post, there are a bunch of different Style keywords you can select from. Ol, which stands for ordered list, will use a number or alphanumeric style, while ul, which stands for unordered list, will use a bullet symbol. If you don't specify the type, ol defaults to 'decimal'. ul defaults to 'disc', a round bullet.
When you want to apply your custom list style, you'll need to apply it using the Styles panel, not from the bullet or number button on the toolbar. These buttons are hard-coded to the built in styles only.
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