According to marsiglia & kulis (2015), the understanding that individuals hold positions within multiple systems of inequality refers to which concept?

The concept that Marsiglia and Kulis are referring to from the statement mentioned in the question is a term called intersectionality.

Intersectionality is a concept that is generally defined as a framework that acknowledges how an individual is impacted by various systems of power - and his or her position in each system, when combined would create a different experience in how an individual would be treated by the society at large.

For example, an individual who identifies as Black, is female, has an upper middle class income, is heterosexual would navigate life differently from someone who is White, male, has a lower class income, and is homosexual.