John Gruber reports on the new version of OmniOutliner (6) which includes a featured called Omni Links:

OmniOutliner has always been document-based, and version 6 continues to be. There are advantages and disadvantages to both models, but one of the advantages to library-based apps is that they more easily allow the developer to create custom URL schemes to link to items in the app’s library. Omni Links is an ambitious solution to bring that to document-based apps. Omni Links let you copy URLs that link not just to an OmniOutliner document, but to any specific row within an OmniOutliner document. And you can paste those URLs into any app you want (like, say, Apple Notes or Things, or events in your calendar app). From the perspective of other apps, they’re just URLs that start with omnioutliner://. They’re not based on anything as simplistic as a file’s pathname. They’re a robust way to link to a unique document, or a specific row within that document. Create an Omni Link on your Mac, and that link will work on your iPhone or iPad too — or vice versa. This is a very complex problem to solve, but Omni Links delivers on the age-old promise of “It just works”, abstracting all the complexity.

I’ve been using OmniOutliner for years, to help structure longer documents and put ideas into some kind of order. Am going to have to try this out, because I’m hoping it will let me link to external documents from within an outline – for example to where I am writing up the thing listed in the outline. I know what I mean anyway.

There’s another Mac app which might do something vaguely similar called Hookmark, which I have never gotten round to checking out properly.