Microsoft appears to be looking to add a Membership category to the Microsoft Store app, next to TV Shows, Books and Movies.
The item was discovered by ALumia and at present appears to show up only when doing searches in Insider versions of the app.
As can be seen from the screenshot above, the category will be populated initially by the Xbox Game Pass membership scheme, but we can easily imagine Microsoft selling other subscriptions such as Office 365 and 3rd party subscription schemes there.
A similar category is already available on the Xbox Console where it shows your Xbox Game Pass and Xbox Live subscriptions status.
Screenshots via ALumia