Basically, the idea is that you assign a named number code to each level, thus creating a named autonumber stream for each level, and you reset that autonumber in the element above it using the code <$rst_numbercode>
So in the Title Prefix of the Section Layout for your subchapters you would have, say (the actual name after the colon doesn’t matter as long as it’s different for each level)
<$n:subchapter>
And in the Title Prefix of the Section Layout for the chapters, you would have
<$n:chapter><$rst_subchapter>
If you wanted to reset the chapter numbers after each change in Part, then the Part Title Prefix would have:
<$n:part><$rst_chapter>
This is from the help item: List of All Placeholders: