Well what I mean by a real category is that people must really think of things in terms of interchangeable units. If they didn’t, then the term ‘supply’ wouldn’t make any sense. So we wouldn’t get very far in economics without assuming that people do really think in terms of different objects being of equal serviceability, i.e. without the concept of indifference. I am using the term commodity to mean “a set of goods with equal serviceability from the point of view of the actor”. Hence I call it a real category. You are defining it as a homogenous class of goods, which is the same thing. So maybe I misunderstood what you meant by convenient abstraction.
So Block would say that when I made a choice to go to the shop and get a sweater, I was considering sweaters as a homogenous class of goods. Then when I get to the shop, suddenly sweaters stop being a homogenous class of goods, and I differentiate them based on their accessibility, choosing the top one. So indifference only applies before the action took place, so is out of scope of praxeology. The fact that I picked that particular sweater was because it was the one on top, hence I was no longer considering all the sweaters to be homogenous at the moment I picked one up.
Hoppe would say that I was considering the pile of sweaters to be comprising a homogenous set of goods throughout the action, even while picking up one of them.
It does seem rather ad hoc to emphasize differences in accessibility, and to consider a homogenous set of goods suddenly “heterogenized” when a choice is made to take one unit and not another. I don’t think this is necessary.
The act of “picking up a blue sweater” or “picking up the sweater on the top of the pile” is just not what Hoppe calls a ‘preferred description’ of the action; the preferred description is “picking up a sweater” since this captures the essence of the action and its purpose, and leaves out details that were insignificant from the point of view of the actor; i.e. that which the actor was indifferent about. The fact that I picked up the one on top of the pile merely reflects me trying to minimize the costs of the action, not a reflection of me suddenly considering the goods heterogenous.