What’s the difference between platform and apps?

I’ve been observing the expansion of this idea of “Ubuntu App Development”.  David Planella just wrote about what the Canonical Community team is doing to build that area. Recently, Daniel Holbach asked if the Ubuntu Packaging Guide should be renamed to the Ubuntu Platform Development Guide (or something similar) since it deals more with working with packages within Ubuntu instead of on top of it.

So here’s my issues and questions. How do we distinguish between apps that we have in the Ubuntu archives (for which the current Packaging Guide would apply) from the apps that people ship on top of Ubuntu (delivered in PPAs, Canonical Partner repos, etc. for which developer.ubuntu.com is maybe the primary entry point)?

I’m personal uncomfortable with the “Platform” language being applied to things like Firefox or Inkscape or the thousands of other desktop and end-user applications. “Platform” to me means libraries and kernels and things like X.org,  awk and curl.

My feeling is Ubuntu is becoming a mix of Debian, where we try to put the entire FLOSS universe in our software repositories, and iOS where we have a fairly clear distinction between Platform and Apps. It’s a bit confusing