SimpleMind updates their apps for macOS and iOS with new feature and enhancements. iOS app can sync in local networks with macOS app without having to sync with a third-party cloud service. This does not work with Android app however. So I guess they use bonjour protocol.
I have looked into other mind mapping software, but I find this app to be more simple, powerful and robust. The UI is smooth and the mind maps produced are even more nice. The themes, customization are also rich enough that I don't miss anything that a mind mapping software requires.
Mind mapping is a visualisation tool for those who prefer having ideas noted down that way. The mobile app helps to quickly brainstorm, visualise ideas and at the end we can see the whole interconnection of topics, which is lot easier to keep in mind than reams and reams of texts.
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.smmx is a proprietary file format for storing SimpleMind mind maps. We can export in other formats like OPML, but it will lose any rich content information, in case one needs to use another app. But I don't see a need for that anyway.