What are point-to-point SMS and off-network point-to-point SMS?

The short message in the network refers to the short message in the same operator's network, such as mobile phone, Unicom phone and telecom phone; Off-network short messages refer to short messages between different operators, such as China Mobile and China Unicom, China Unicom and Telecom, Telecom and Mobile. No matter the short message inside or outside the network, the message receiver is always free.