On iOS devices (iPad, iPhone, iPod touch) apps can be launched if they subscribe to a defined URL scheme. This is also known as 'app switching'.

iViewer 5 app can be launched via URL schemes, same as iViewer 4 below, just change iviewer4: to iviewer5:.

Use this to launch iViewer 5 with a specific GUI URL and settings:

iviewer5:///gui/?url=<encoded_url>&reloadGUI=1

The encoded_url must be the URL you want iViewer to load, in URL Encoded format, eg: to load a GUI at http://192.168.0.10:8019 the full URL to use is:

iviewer5:///gui/?url=http%3A%2F%2F192.168.0.10%3A8019&reloadGUI=1

Other options you can use in the URL are documented in the CF.loadGUI JS API

iViewer 4 app supports being launched via URL Scheme as follows:

Use this to launch iViewer to a specific page designated by pagename:

iviewer4:///page/pagename

Use this to simply launch iViewer to the startup page:

iviewer4://

iViewer Lite app supports being launched via URL Scheme as follows:

Use this to launch iViewer Lite to a specific page designated by pagename:

iviewerlite:///page/pagename

Use this to simply launch iViewer Lite to the startup page:

iviewerlite://

iViewer next (beta) app supports being launched via URL Scheme as follows:

Use this to launch iViewer to a specific page designated by pagename:

iviewernext:///page/pagename

Use this to simply launch iViewer to the startup page:

iviewernext://

CF iViewer (legacy) app supports being launched via URL Scheme as follows:

Use this to launch iViewer to a specific page designated by pagename:

iviewer:///page/pagename 

Use this to simply launch iViewer to the startup page:

iviewer:// 

From iViewer, you can force other apps to be launched on Button press by assigning the URL Property of the button to a specific URL Scheme for the app you want to launch. Note that not all apps can be launched. The developer of the app must enable the feature and assign a special URL Scheme for their app to allow it to be launched from other apps.

iViewer will then either close or go to the background (depending if you have multitasking enabled in iViewer settings or not) whilst the desired app is launched.

A common request is the ability to launch the Apple Remote app. For this, use the following as the button URL property:
remote://

Sonos App is another common request, and they have implemented the following URL Scheme:
sonos://

For other apps, try a Google search to find the URL Scheme for the specific app you need to launch. Or try contacting the app developer to find the information first hand.

This website has a searchable list of URL schemes which may be of use: http://www.gotschemes.com

You can also try this guide for finding the URL scheme of any iOS app: https://www.amerhukic.com/finding-the-custom-url-scheme-of-an-ios-app