question #1 Sahih Muslim Introduction 84, Introduction, Narration 83 : What is the difference between backbiting and assessing someone’s reliability in Islamic teachings?

‘We were near Ismā’īl bin Ulayyah, and a man narrated on authority of another man, so I said: ‘Indeed this is not reliable (Thabt)’. So the man said: ‘Are you backbiting him?’ Ismā’īl said: ‘He is not backbiting him; rather he is judging him unreliable’.