This counts how many times the switch has had to resend an Access-Request to this RADIUS authentication server because the original attempt wasn't answered in time. A high retransmission count relative to the Requests counter on one server, while other configured servers look normal, is a specific signal of network congestion or an overloaded RADIUS process on that one box rather than a client-wide problem. For example, 300 retransmissions against only 320 original requests would indicate this server is answering barely any request on the first try, likely due to a saturated CPU or a slow backend datastore on the RADIUS server itself.