Custom messages are similar to one-off system messages with one important difference:
Custom Messages are fully customizable messages that you can send at any time and for any reason. Read this article for more info.
One-Off Messages are system generated messages you can customize that Gazelle will send in response to specific situations (For example: when a customer schedules an online reservation, confirms an appointment, or when you send a customer an estimate, invoice, and/or approve/decline their online reservation). Read this article for more info.