Could anyone please tell me what protocol does OSIsoft PI AF Transformer use for performing its operations?
From the release announcement AF Transformer 18.104.22.1686 is released one of the components is PI AF Client(PI AF SDK).
More information can be found at "Hello World" In AF Transformer To Pivot Your AF Database (Part 1)
For low-level interaction with AF and PI Data Archive it's using AF SDK. Above it is an intelligent data translation layer that OSI uses to rapidly build data transformations with (which is what you're given exposure to). It can save you from writing AF SDK code for the trade-off of editing XML directives instead.
Retrieving data ...