It shares the data of the devices on the same bill that are on the same family of plans. So if you had 3 devices, each on a plan that has 20GB, you would have a pool of 60GB to use across those devices. Handy if you have someone who doesn't use much data and someone else who does.
Just make sure that you read what is included in those plans before you sign up. The inclusions are different to the mainstream plans.
Never be afraid to back yourself when trying new things, just always make sure you have 3 escape routes if things go wrong.
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