# What is the definition of function index

I'm reading through Indistinguishability Obfuscation from Well-Founded Assumptions and in Definition 3.1 describing sPRG, it mentions "samples a function index I." Can someone explain what a function index is in this context?

A sPRG as defined there describes a family of functions, i.e. a set of many functions $$f_I$$. The index $$I$$ identifies one such function from the family and the algorithm $$\mathsf{IdSamp}$$ describes how you would sample such an index. The $$\mathsf{Eval}$$ algorithm given $$I$$ and $$\mathsf{sd}$$ then evaluates the function $$f_I$$ on input $$\mathsf{sd}$$.