Plans are for your own personal use on one phone. Do not resell, rent or share one, or use it to provide a service to other people.
Nothing illegal, no spam, and no attacking networks or devices. No automated or machine-to-machine traffic beyond what a phone in normal use generates, unless the plan says it is allowed.
Do not try to get around fair-use limits, speed caps or the coverage area. A plan used in breach of these rules can be suspended or ended, because the networks we buy from require that.