A scheduled message is one queued to send at a chosen future time, or within a delivery window, so a reminder lands the evening before an appointment rather than the moment it is created.
How scheduling works in practice
The RBM API itself sends immediately; scheduling is a platform feature that holds the message and submits it at the set time. Good platforms schedule in the recipient's local time zone, which matters in the US where a 9 a.m. send from New York arrives at 6 a.m. in California, and let a whole campaign be staggered across a window rather than fired at once.
The compliance angle
Scheduling is how quiet hours are honoured. The TCPA's telemarketing rules and the CTIA's messaging principles keep marketing messages to reasonable hours, and several states set their own windows; a scheduled send is the mechanism that makes "no marketing before 8 a.m. or after 9 p.m. local time" true for every recipient. Transactional messages such as fraud alerts and one-time passcodes are the exception: they go when the event happens.
Pairing it with a validity period
A scheduled message should carry a validity period so that a reminder for a 3 p.m. appointment does not arrive at 5 p.m. because the phone was off. SimplyRCS blasts schedule per time zone and drop a message that has expired unsent instead of delivering it late.