Do you mean how was it rationalized during the middle ages? It was not, because as is explained at length in the FAQ and by my own post here, modern historians no longer give weight to the idea of Feudalism (or at least say that they don't). Instead, they see it as a construct of the 17th century.

Now, how the 17th through 21st centuries used the (false) idea of feudalism to rationalize their economic systems - that is an intriguing question, and one which I try to answer in the post linked above.