Like the difference between answer rate and contact rate, engagement rate is the percentage of calls where the callee talked for at least 60 seconds (or your Successful Call Duration). Meanwhile, success rate is the percent answered unique called numbers with a talk time of one minute. This tells you how well you’ve completed your calling list.
If you called two people, and person A answered and talked for 1+ minutes, but person B answered and only talked for 30 seconds, your engagement rate and success rate would both be 50% (1/2 calls were answered where a number talked for more than 60 seconds AND 1/2 called numbers had a call which was answered and they more than 60 seconds).
However, if you called two people and person A answered and talked for 1+ minutes, but person B did not answer so you called them a second time and they still did not answer, your engagement rate would be 33% (1/3 calls talked for more than 60 seconds), but your success rate would be 50% (1/2 called numbers had a call that talked for at least 60 seconds).
For more information, check out the INFORM Reporting Metrics & Definitions FAQ.