The Subject numbers are unrelated to the person numbers but are sequential going down? Correct.
For those, I would use a SEQuence Field.
The person numbers would be a level 1 number in a multilevel list.
The 1a,1b,2a,2b numbers would be a level 2 number in a multilevel list.
If you use automatic paragraph numbering or bullets read Shauna Kelly's directions on numbering and bullets. Start with How to create numbered headings or outline numbering in Word 2007 and Word 2010. For large documents you must follow these directions or you will lose your hair! This is the same link that John provided.
(Mac version: https://www.brandwares.com/bestpractices/2016/06/outline-numbering-in-word-for-os-x/ This is John's translation of Shauna Kelly's instructions for the Mac interface.)
This may seem a bit convoluted at first, but it really is not. Just follow the steps. Shauna Kelly's instructions use the built-in heading styles, but you can use any existing paragraph styles including your custom styles. Note, you can modify the built-in styles to look exactly the way you want.
If you want, I can provide links to videos demonstrating setting up numbering linked to styles.
The basic idea is that the numbering is set using the Define MultiLevel List dialog with each numbering level being attached to an existing paragraph style. Once you have this set up, you should not use the buttons for numbering in the Ribbon but rather apply the appropriate style for that level.

- The screenshot shows heading levels being used but any paragraph style can be used.
- A unique style for each level.
- The level 2 was told to include the level number from level 1.
- The alignment was set at 0.
- The Text Indent may no matter but was set at .25.
- A tab stop was added at .25 with the number to be followed by a tab character.