Tex27 is regular textile thread which is used all around for pajamas, shirts, dress pants and suits, nylon jackets etc. I do not have any smaller threads than this and do not remember ever coming across those.
Tex40 is heavier weight textile thread, used for some denims, cargo pants, uniforms etc.
Tex70-90 are often used for jeans and other denim items, work pants and jackets. I make and hem jeans with this all the time.
Tex110 is super-heavy thread used for safety belts/harnesses, tarps, and for accents on designer clothes. This is the largest textile thread I ever came across.