When you download a widget into your project Mendix and add it onto a page the Mendix platform will check the offlineCapable setting. If the page is an online page then it will accept both offline and online widgets. If it's an offline page it will only allow offline capable widgets.
We also try to indicate using appstore pictures that the app is offline:
I do think it is an area we could make more obvious via our AppStore.
One of the biggest limitation of offline widget that they can not access offline data. With the latest release Mendix 7.6 and 7.7 this limitation is taken away. Without this limitation many widgets could be made offline capable.
> Widget Improvements
> Entity paths for custom widgets in offline apps are now allowed! This means you can use custom widgets that have an entity property in your offline pages.
> Offline Data Retrieval for Custom Widgets
> We have created a new API for custom widgets to fetch data while being offline:
> It supports data filtering, sorting, and paging. Please note that offline apps currently download and synchronize only that data which is used by some of the “core” UI widgets.