Delivery rate is the percentage of sent messages that the network confirmed reached the recipient's device, counted from delivery receipts rather than from what was submitted.
Sent, delivered and the gap between them
A platform accepting a message is not delivery. The message can be rejected by a carrier's spam filter, sit undelivered on a phone that is off until its validity period runs out, or fail because the number is a landline. Only a delivery receipt, a mobile originated event that the handset confirms receipt, counts. RCS returns one for every message; SMS returns one on most US routes.
What moves it
Registration is the biggest lever for the SMS side: unregistered 10DLC traffic is throttled or blocked, and a low trust score caps throughput. List quality is next: a number lookup removes landlines and dead numbers before they drag the rate down. For RCS, delivery to a capable handset is close to certain; the variable is how many handsets were capable, which fallback handles rather than failing.
How to read a number
A delivery rate is only comparable when the denominator is stated. "98% delivered" over messages accepted by the carrier and "98% delivered" over messages submitted are different claims. SimplyRCS analytics count from submission and show RCS and SMS separately. See also deliverability.