You will see 'Add-On' on your plan if you:
1. Signed up for the 100GB Plan: This plan is made up of the 20GB Base Data and Unlimited Data* Add-on (which is added by default). You will see Unlimited Data Add-On in your invoice as long as you are on this 100GB plan
*Your Unlimited Data consists of 80GB 4G speeds + Unlimited Data on throttled speeds afterward.
2. Manually added the Unlimited Data, IDD Calls, Unlimited Outgoing Calls, or Unlimited Incoming Calls to your plan during your sign up
Also, please note that your data allocation in the first month will be pro-rated. The 20GB Base Data is pro-rated to the date your number is successful. The 80GB will not be pro-rated. Hence, your full data allocation for your first month will be between 80GB - 100GB.