Or may they also be used for optional features?
The question Desktop Publisher (Alternatives to InDesign) is tagged with free and open-source (which would be valid combination, as Open-Source software must not be available for free). But in the question it says:
Cost/Licence: Must be free, Ideally would be Open Source
(Same case with the question Free alternatives to Matlab that run on Linux?.)
The question Free reference management software with full-text support is tagged with windows and web-apps. But in the question it says:
Target environment: Windows desktop or web
So here not both must be true (web-apps running on a Windows server; or being available cross-platform), but only 1 of 2 is required.
The question Music manager for a mass storage based music player is tagged with windows and linux. But in the question it says:
I'm running Windows 7 and 8 mainly, so I would prefer windows, but I would accept a Linux based solution if nothing else is as good.
Here the software doesn’t have to be cross-platform, it’s either Linux or Windows (or both).