A promotional (marketing) SMS advertises a product, offer or event to people who agreed to receive such messages; it must carry an opt-out and respect local quiet hours.
Marketing SMS is any message whose purpose is to sell or promote. It needs prior consent from the recipient in almost every jurisdiction, an easy way to stop (a STOP keyword or a link), and delivery inside the destination's permitted hours. Frequency matters too: two to four messages a month keeps lists healthy; daily blasts burn them.
Carriers filter promotional traffic that looks unsolicited, and regulators fine senders who ignore consent. The cost of a careless campaign is paid on every later one, because opt-outs are permanent.
At SMSMeteor: quiet hours apply by default per destination, STOP handling removes numbers from future sends, and the acceptable use policy prohibits harvested lists.