The "OSM Tasking Manager" is run by a private organisation in the US, HOT Inc. which prefers to propagate a "slightly" different on-boarding model than customary with the rest of OSM (taking baby steps till you feel comfortable with editing). Which is why they have to validate edits and need to steer contributors away from tasks that may not be suitable (in the original OSM model this is not necessary).
In summary you would need to ask HOT what which criteria they are using to grade mappers: https://www.hotosm.org/contact-us The system definitely hasn't been used before in OSM and is not an existing standard.