Private Credit Secondaries - Valuation techniques

Hello All,

I am currently looking to move into secondaries, some of the places that I have looked at are focussed on private credit investments. I understand that the pricing of these transactions are represented as a discount to NAV of c.15% or so (depending on various factors like distress situations, reason for sale, and so on). 

Please could someone explain to me how the valuation is carried out in a technical sense, what models are used? If similar models are used than seen in other areas of finance, how are models for secondaries private credit distinct? Also, are there any good resources for learning private credit specific valuations [for secondaries]? (I have struggled to find many good resources on the topic)

Will massively appreciate any advice - thanks!