For example, Marx says in the Critique of the Gotha Programme, describing the situation under Communism,
"Here, obviously, the same principle prevails as that which regulates the exchange of commodities, as far as this is exchange of equal values. Content and form are changed, because under the altered circumstances no one can give anything except his labour, and because, on the other hand, nothing can pass to the ownership of individuals, except individual means of consumption. But as far as the distribution of the latter among the individual producers is concerned, the same principle prevails as in the exchange of commodity equivalents: a given amount of labour in one form is exchanged for an equal amount of labour in another form."
A primitive Community that produces Use Values for their own consumption, does not produce commodities, but does produce Use Values that have Value, because they have required human labour to produce. The Value for this community of producing 20 yards of linen is expressed in the fact that, it has had to forgo the coat it could have produced instead. How much Value a society places on a particular Use Value can be gauged by the proportion of social labour-time it devotes to its production.
The orthodox economics equivalent of Value is Opportunity Cost in Production
As Marx says in Capital, Value is both logically and historically prior to Exchange Value. Use Values that have been created by Labour, are termed "products" by Marx, and possess Value prior to becoming commodities, and prior to having Exchange Value. Exchange Value can only arise when all of these Values are brought into a social relation one with another through Exchange.
Similarly, products produced by a Socialist economy will not be commodities, but will have Value. As Marx puts it, in fact Value will be even more significant then.
“Secondly, after the abolition of the capitalist mode of production, but still retaining social production, the determination of value continues to prevail in the sense that the regulation of labour-time and the distribution of social labour among the various production groups, ultimately the book-keeping encompassing all this, become more essential than ever.”