This is anything that will help you visualize your character moving in space.
These can include: age, height, weight (or body type), eye color, hair color, skin color, scars, tattoos, left/right-handedness, verbal tics, inheritable conditions (color-blindness, diabetes, high blood pressure)
(2) Social markers.
These "markers" are social constructs, and carry meanings that are particular to each time period and location.
They include: race/ethnicity, gender, nationality, class, sexual orientation, education, dis/ability, religion. As a sidebar, I recently listened to a talk by K. Tempest Bradford on intersectionality, and she makes the point that social markers such as race and gender and disability inflect each other. This may be something to consider as you’re developing your MCs or SCs. I have a short essay elsewhere under the FOR AUTHOR tab about this, if you’d like to know more.
(3) Personality traits
I’d say pick 5 for now, when you're starting. Generous, selfish, anxious, ambitious, sly, naïve, fussy, an insomniac, impatient, insightful, empathetic, shy …
In our Western culture (again, these are era and place specific) these "shorthands" gesture to what is almost a small plot-line, or a set of traits that we might expect.
Examples are: divorced seven times, only child, orphan, ex-Marine, Depression-survivor, cat-lover