I've never seen any documentation for the subject tables (or the "selected population profiles", which technically, is what S0201 is, despite having a "table code" that makes it look like a subject table)
However, just looking at the numbers for the percentages under "all people", one has to guess that the denominator is roughly "of all people of the same age", not "of all people" or "of all people in poverty". (I roughed that out comparing B17001 and B01001 )
I'd love to see systematic metadata documentation, and it seems like it could be integrated into the API system with the "variables" metadata, like https://api.census.gov/data/2019/acs/acs1/spp/variables/S0201_262E.json