The following resources will help you better understand the differences between tournament levels 1-7, the junior circuit (net generation), 10 and under tennis, rankings and changes to the 2021 tennis year.   

10 and unders - Net Generation

Tournament levels 1-7 - Format and Comparison

Tournament levels 1-7 - Ranking System

USTA junior tournaments - Changes for 2021

Level 7 tournaments - More Info

How to withdraw from a tournament:

https://customercare.usta.com/hc/en-us/articles/360052789192-How-to-Withdraw-From-a-Tournament