The general notation for your subtopics in an outline is

The question is asking to state what could be the general notation for the sub topics in an outline and base on my research and further investigation, the possible answer would be roman numerals for the top level, upper-case letters (in the alphabet of the language being used) for the next level, Arabic numerals for the next level, and then lowercase letters for the next level

Answer;

Capital letters.

Explanation;

According to Writing with Power, Capital letters are the general notation for the subtopics in an outline.

The general notation for your supporting is an outline is arabic numerals.

Roman numerals on the other hand are the general notation for the main topics in an outline.