I find the OECD's levels of literacy more instructive than grade level. This page has a short definition: https://literacytrust.org.uk/parents-and-families/adult-literacy/what-do-adult-literacy-levels-mean/

pdf page 75 (table 4.5) here has more detailed definitions: https://www.oecd.org/content/dam/oecd/en/publications/reports/2019/11/the-survey-of-adult-skills_d7f1bc16/f70238c7-en.pdf