Sonatype CLM Server - Policy Management Guide
In the case of organization level policy (rules), which appears across many different applications, the application level policy is meant for precise scenarios where the policy is only intended for a single application.
Doing this takes into account something specific we want to identify or keep out of a single application, but not others. The more of these application level policies that are created though, means the more micromanagement, and in turn, opportunity for error, will occur. So, keeping them at a minimum, and only for those unique scenarios is ideal.
This is likely better conveyed in an example. Imagine two applications with 4 policies each. Two policies for these applications are identical. If you have this setup with two separate applications, any change to the identical policies has to be done once for each application. However if we move these policies to the organization that both applications belong to, we only have to change one. Now imagine a similar scenario with a larger number of shared policies as well as applications. Without organizations to inherit from this would become unmanageable quickly.
Alright, we understand organization level policies are a good idea, and application level policies should be used as minimally as possible. This tends to create a question of, "How do I know when to create an organization vs. application policy?" In reality the first limitation will actually be related to what you have access to create and/or view.