Comment by bdash

It is a little more direct than that even. The application's entitlements are passed into the interpretation of the sandbox profile. It is the sandbox profile itself that determines which policies should be applied in the resulting compiled sandbox policy based on entitlements and other factors.

An example from /System/Library/Sandbox/Profiles/application.sb, the profile that is used for App Sandboxed applications, on my system:

  (when (entitlement "com.apple.security.files.downloads.read-only")
        (read-only-and-issue-extensions (home-subpath "/Downloads")))
  (when (entitlement "com.apple.security.files.downloads.read-write")
        (read-write-and-issue-extensions (home-subpath "/Downloads")))
  (when (or (entitlement "com.apple.security.files.downloads.read-only")
            (entitlement "com.apple.security.files.downloads.read-write"))
        (allow process-exec (home-subpath "/Downloads")))