General feedback
There's no merge request to comment on, so I'll place links to source lines inline.
https://source.puri.sm/pureos/infra/pureos-policy/-/blob/main/index.rst?plain=1#L4 What's PureOS and PureOS Store? Ideally, someone sent here by a search engine will not get lost.
IMO a whole section is missing: who is this documet for and why should I care? This should be sth like: "PureOS Store (link) is a place where developes like you publish their applications, so that PureOS (link) users can (optionally pay and) start using them on compatible computers (link) and phones (link). We will publish your app (link to submission tutorial) (and handle your payments (link)) as long as it meets the requirements (link). Anyone is allowed submit an app to PureOS Store, except (... US sanctions against Iran).
"Flatpak" should be capitalized, as it's a brand name. This will reduce ambiguity to those who aren't famliar with the term.
Also, Flatpak is many things: the Flatpak application packaging format, the application download protocol, both together, and also a concrete application package. In most cases, it's about the packaging format, but in Getting Started https://source.puri.sm/pureos/infra/pureos-policy/-/blob/main/get_started.rst?plain=1#L4 it's the download protocol.
https://source.puri.sm/pureos/infra/pureos-policy/-/blob/main/get_started.rst?plain=1#L14 - why? (please ignore this remark if you're planning to fill it in)
https://source.puri.sm/pureos/infra/pureos-policy/-/blob/main/requirements.rst?plain=1#L4 "for the most part" should be clarified (please ignore if TODO)
https://source.puri.sm/pureos/infra/pureos-policy/-/blob/main/requirements.rst?plain=1#L6 Free Software deserves a link to the binding definition (I know it's linked later, but the first reference should generally have the link; e.g. Wikipedia does this)
https://source.puri.sm/pureos/infra/pureos-policy/-/blob/main/requirements.rst?plain=1#L6 what is `sm.puri.Platform`
?
https://source.puri.sm/pureos/infra/pureos-policy/-/blob/main/requirements.rst?plain=1#L9 Passive voice is hard to read. Instead sth like "PureOS and PureOS Store only accept an app for inclusion when the app fulfills those requirements:"
The above is kind of awkward, because it's not been made clear yet: who is allowed to fill PureOS with apps?
https://source.puri.sm/pureos/infra/pureos-policy/-/blob/main/requirements.rst?plain=1#L11 I'm not sure this is necessary. Developers coming from other dstribution systems may react badly to that, and even the GPL only requires that the source code is available to those who get the binary. On the other hand, it may be too much effort for us to ensure sufficient distribution, so "public" is the easy solution. Anyway, I'd suggest: "Source code must be available to Purism, and to every user who downloads the corresponding software".
https://source.puri.sm/pureos/infra/pureos-policy/-/blob/main/requirements.rst?plain=1#L14 what does it mean "have"? Or "app"? It's useful here to define what is considered the app - a source snapshot for the Flatpak, or the actual Flatpak package, or the submitted repository.
https://source.puri.sm/pureos/infra/pureos-policy/-/blob/main/requirements.rst?plain=1#L18 IMO this is needlessly exclusive. We're already fighting an uphill battle against other attractive platforms, no need to alienate developers even more. If it's only "should", then it should not be in "requirements" but in "suggestions" instead.
https://source.puri.sm/pureos/infra/pureos-policy/-/blob/main/requirements.rst?plain=1#L17 - I'm not familiar with Flatpak, but is this actually a requirement? What about something written in SDL?