Marx used “class” in “two different ways” (Calvert).
Some sources—like an ebook—employ location indicators other than page numbers. If your work is divided into stable sections like chapters, those sections may be cited. Do not use the location numbers used within an ebook as the locations are often specific to that app or device.
Marx used “class” in “two different ways” (Calvert, ch. 4).

Marx used “class” in “two different ways” (Calvert).
Some sources employ location indicators other than page numbers. For example, an ebook may include a numbering system different than page numbers. If your work is divided into stable sections, those sections may be cited.
Marx used “class” in “two different ways” (Calvert, par. 4).