For HTML-based output, if you are not using CSS, the list style property does not apply. Instead, DITA2Go uses the format component assigned to property number; see Table 7-3. The list style property is for CSS, and determines what an ordered or unordered list looks like. List styles do not produce anything in RTF output.
The number property, in contrast, produces an autonumber. Autonumbers work in Word and also in HTML, outside of lists; within HTML list styles, DITA2Go turns off autonumbers by default.
It is a good idea to set both number and list style, so that your formats are covered whichever way you go. You will not get duplicate numbering if you specify values for both list style and counter types. For example:
[Numbered2] based = Numbered list style = lower-alpha number = List2Num
[List2Num] stream = ListStream counter = 2 form = <tab/><counter2/>.<tab/>
[ListStream] counters = 3 counter types = Num LCAlpha LCRom