A few hints for data usage when using a Wi-Fi hotspot:

• To your device (phone, tablet, computer), the mobile hotspot will look exactly like any other wifi connection you may use.
• Any settings for App downloads or podcast downloads that are configured to "automatically download" when connected to Wi-Fi, will use the mobile hotspot Wi-Fi connection to download apps, iOS or Android OS updates, or podcasts and will use your data plan.
• Consider turning off automatic backups - the backups will use the mobile hotspot to send backups to iCloud or other cloud-based backup services.
• The GB shown on the mobile hotspot display is the amount of data used, not the amount remaining.
• Turn off your device overnight when not in use.