BubbleUPnP Server is a piece of software that can add features to servers and renderers alike, whereas '
BubbleUPnP' is a control point for Android.
With BubbleUPnP Server installed you can turn your renderer/Chromecast into a DLNA or OpenHome device so it'll work natively with other DLNA/UPnP control points. In the
interface you simply choose your rendeder and then tick the 'Create an OpenHome renderer' and in your control point you'll see the renderer named with a suffix of '(OpenHome)'.
Once you've chosen the OpenHome renderer in your control point, it's essentially talking to BubbleUPnP Server as though it were a native renderer, which then passes the commands onto the actual renderer/Chromecast itself.
The other main benefit of using BubbleUPnP Server is that the playback queue is held (and advanced) on the server, and not on the control point, so you don't need your phone awake to advance the tracks, and you can access the queue from other devices. This is why, once you've got it working I expect you'll turn all your devices into OpenHome renderers :-)
The only limitation of using the OpenHome functionality is that the control point also needs to support OpenHome, so something like Lumin should work on ios.