@Gaurav Tewari 

I can rephrase your question as - when should a Resource be modelled as a Resource Spec versus a Configuration Item?

A Resource in the context of P-S-R model, represents a tangile or intangible entity required to deliver a Service and where the Resource solely belongs to the customer such that it's lifecycle is tightly coupled with the customer. If the customer decides to disconnect, the Resource instance is also deactivated.

A CMDB CI is Resource+. By that, I mean the CI can represent a Resource that solely belongs to the customer but it can also represent a shared Resource instance that is common across different customers and serves more than one Service.

Based on this, a Resource can be represented in 3 ways -

[1] Only as a Resource Spec, and the instantiated record are stored in the Product Inventory table.

[2] Only as a CI, which can be linked to the Service Spec via Install Base Item

[3] Both as a Resource and CI

Pattern 2 and 3 are useful in Service Impact Analysis and Assurance use cases.

Based on your needs, you can either follow pattern 2 or 3.

You do not need TNI for this.

You mention, if you model the Resource as a CI then modelling CSF/RFS is a challenge. This I do not understand. Can you elaborate?