A de-registration delay refers to a period of time from 1 to 3600 seconds where your LB will allow in-flight, on going, requests to complete before the machine is deregistered, most likely for being in an unhealthy state. During this period new requests are not accepted but requests that have already gone through will have until the end of the de-registration period before the instance is de-registered. A key example for why you’d want a longer delay is perhaps there is an ongoing file upload to the instance you’d most likely want to allow that the time to complete.