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