Susan Reynolds argued in Fiefs and Vassals that the "feudal system" is largely a construction of later lawyers. What is the core of that argument?

C. The terms fief and vassal are used loosely and inconsistently in the medieval sources themselves

Her method is philological: read the documents rather than the textbooks, and the neat vocabulary dissolves into local variety. Land was certainly held in return for service, that is not in dispute. What is disputed is whether contemporaries had, or needed, a single coherent system of tenure, or whether academic lawyers built one afterwards and historians adopted it.