What's the difference between short messages in the network and short messages outside the network?

For example, if it is a mobile phone, the short message sent to the mobile phone is an in-network short message, while the short message sent to Unicom or PHS is an off-network short message. If you use SMS package, some SMS messages inside and outside the network are counted separately, depending on the type of mobile phone card used. For example, Beijing M-Zone, which used to be counted separately, was just cancelled in August, and all short messages inside and outside the network are counted in the short message package.