Pacing applies to marketing and utility templates. According to Meta's documentation, newly created templates, templates that were paused and unpaused, and templates without a high quality rating can be paced. Messages go out normally until an unspecified threshold, then the following ones are held while Meta collects feedback.
If the feedback is positive, the held messages are released and the send reaches the full audience. If it is negative, the template status becomes paused and each held message is dropped, with error code 132015 in the API. Meta states that its goal is for paced campaigns to be delivered within an hour.
Pacing rewards the same habits as every other quality mechanism on the platform: a clean, opted-in list, a strong first line, and relevant targeting, so the first wave of recipients votes in the campaign's favor.
