Discussing the documentation I produce with a coworker recently reminded me most people have never heard of outliner software.

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Outliners are word processors specifically built to edit hierarchical text. I use this one to record and organize notes from meetings, plan involved changes, build detailed runbooks, and more. In the document open in the screenshot, I'm planning what I want my coworkers to build in a lab environment. Later, I have a list of the things I want to test, which I plan to elaborate, as I have time.

One big advantage is you can collapse sections you don't actively need at the moment. For example, in my runbooks, I build very specific categories of change. That way, for multi-part changes, you can open the runbook and just collapse all the sections which don't apply to you. If it is built well, you wind up with a series of steps which cover exactly the change you are trying to make, all from one generic document.

Microsoft Word has an outliner mode. There are also dedicated programs like OmniOutliner, which tend to have more features specifically applicable to hierarchical text.