Marxist criticism and Cultural criticism address what is not in the text, what the text hides or neglects to say. Compare this to Structural criticism that addresses what is exactly in the text: the diction, the irony, the tropes and the rhetorical devices etc. Both Marxism and Culturalism seek to expose the reality that underlies the narrative without being explicitly stated in the narrative. Marxism looks for the exploitative economic underpinnings of the narrative, while Culturalism looks for the social underpinnings of the narrative.
Marxism asks questions like: what is the economic base? what is the superstructure of law, philosophy, politics etc that superimposes order on the economic base? who is heard and who unheard in the base? what is the ideology of beliefs and values that goes unquestioned in the narrative yet guide the actions and thoughts of the actants in the narrative?
Culturalism looks for the web of social forces that create the cultural moment captured in the narrative, though perhaps only implied in it rather than explicitly illuminated in it. Culturalists ask questions like: what are the social-cultural discourses that attribute power and, conversely, powerlessness? what are the discordant, subversive sociocultural discourses? wherein lies the challenge to the legitimate voices of socio-cultural discourse from the unheard, unrecognized voices?
Though there is little space available, there are a great many examples of all these questions in Austen's Emma. Starting with the last question, the gypsies are a denied voice in discordant, subversive discourse they present: socio-cultural discourse would make them a banned other, neither seen nor heard, yet they assert their right to be. Emma, a voice in the legitimate discourse, objects to Jane's "reserve," demonstrating a perceived challenge to the socio-cultural mandate of "precedence," or social superiority and status.
Skipping to the Marxists questions, the dominant ideological belief--the predication of the novel, which both Austen (through implication) and Knightly challenge--is that status in the base economy has the right to manipulate the existence of those without status in the economic base. This applies directly to Emma's manipulation of Harriet and directly to why Emma's "real evils ... [are] were the power of having rather too much her own way, and a disposition to think a little too well of herself."
One last example: Who is heard and who unheard in the economic base presented in the narrative? We already know who is heard: Emma (and Knightly, Mr. Woodhouse, the Churchills). Who is unheard? Who does the economic base and superstructure render invisible and powerless? In this narrative, that is the servants. We know there are servants. Who else prepares the gruel Mr.Woodhouse relishes and serves at the dinner and card parties Emma organizes? Yet, in the narrative, and presumably in Austen's own world, the servants are an invisible, powerless element in the economic base and in the socio-cultural consruct.
Other things to consider in Emma are Emma as economically disempowered though the heroine; Harriet's position in the economic base; Mr. Knightly's challenge to the base through his behavior toward Mr. Martin and Harriet; Elton's socio-cultural assumptions; Miss Bates' socio-cultural role; and the import of both Frank and Jane being raised elsewhere in families not their own.
I suppose Colonel and Mrs. Campbell will not be able to part with [Jane] at all.