I have always understood a (broadly speaking) capitalist, free market approach to economics to rest on the notion that a resource distribution is fair in light of how closely it correlates the resources an individual or firm receives to the value they contribute to the market. This value might be apparent and concrete, such as a good, service, loan, land utilization, labor, etc., or abstract, such as human attention, a vivid vision of a firm's future operations and role in the economy, liquidity, etc.
On the flip side, I have understood a (broadly speaking) socialist approach to economics to rest on the notion that a resource distribution is fair in light of how closely it correlates the resources an individual or firm receives to his/her/its need for such resources. For example, August Becker described "From each according to his ability, to each according to his needs" as "the basic principle of communism."
However, in order to ground normative claims related to the bourgeoisie/proletariat distinction, perhaps most notably the claim that the hierarchies they form and occupy in a capitalist economy are fundamentally unjust, socialists seem to make frequent use of the former notion of fairness, perhaps merely failing to recognize some of the categories listed above as legitimate value (claiming that CEOs, landlords, securities traders, etc., "contribute nothing to society"), but generally recognizing at least labor, goods, and services as legitimate things to correlate to the acquisition of economic resources. Whether or not you think this is a narrow minded analysis of what contributes to society, at the least it is an attempt to correlate resource distribution with contribution to society, as opposed to with need. Thus I am confused as to what notion of fairness a socialist economic theory actually rests upon.
Obviously, pretty much everyone agrees that a fair resource distribution is not automatically a good resource distribution. No one getting any resources is presumably fair, but not very good. The goodness of a resource distribution is thus a multivariable function of its fairness and some other consideration(s).
Does the socialist fundamentally disagree with the capitalist about what constitutes a fair resource distribution, or is the foundational notion of fairness agreed upon, with the socialist merely giving less weight than the capitalist to fairness relative to the other considerations that factor into crafting a viable resource distribution model?