The 'SMS Subscription Settings' enables you to toggle between transactional and marketing messages.
Based on which option is set, messages can be sent to different sections of the subscriber base depending on their opt-in status. Marketing messages require explicit opt in consent (such as from those customers who have checked the marketing box at checkout or who have proactively messaged your brand number). Transactional messages are those such as a confirmation message that update the customer on the status of their order. Transactional messages do not require explicit consent and so can be sent to all contacts, regardless of whether they have opted in to receive marketing communications.