Hi @fern1,
I definitely have had multiple "dev" suites attached to my mobile apps at one point (when we were starting our apps, I wanted a "clean" suite in which to play around with processing rules and testing data in isolation of out global suite set up.
I think the reason it's not allowing you to add your website suite is just a matter of configuring your suite to accept mobile app data. There should be instructions for how to turn that on from the link that was provided in your screenshot.
This leads me to an aside (I will get to your last question next), but not to throw a wrench into your plans and setups... but have you considered just using one global suite, and creating virtual suites?
This is how I have our systems set up... I have 31 "brands" (31 core websites, each brand has multiple "extended content" sites, and 3 of these brands has a mobile app).... all feeding into one global suite. I have a virtual report suite for each brand's "core website", I have virtual suites for "all extended content", I have virtual suites to group some of the extended content together for that particular business unit who want to see that content aggregated, I have a virtual suite for mobile apps, etc....
This way I can see all the data together, or break it out to the individual piece that I need... and I am no longer incurring the cost of multiple server calls for tracking going to multiple suites.
This significantly reduces tracking complexity and costs.. and at the end of the day, the decision is yours to make.. but it is something I recommend considering.
Now, back to your last question about tracking people who go from the app to the website.
You can use visitorAppendToURL (https://experienceleague.adobe.com/docs/mobile-services/ios/sdk-reference-ios/hybrid-app.html?lang=en there is a similar code for Android as well) to append the user's ECID to the web call, that you can then use to properly map the app user's ID to the website. If you want to add additional "campaigning" you can.. but I would probably avoid UTMs as that is reserved for "external" campaigns... I would be more likely to go with an ITM (internal tracking campaign) and creating specific tracking around that... but asking your devs to append this to every url may get troublesome....
I am not sure the nature of your app... when you click on a web link... is the website opening within the native app as a webview? Or taking people right out of the app and into their default browser?
If the first, then you can actually do something else entirely! I had our developers follow a similar implementation that many social media apps are using, and I had them modify the app's User Agent string by appending a recognizable item to the end....
So let's use the Facebook app for instance...
When a user opens a webpage within their webview, the user agent string users the user agent of the default browser on the device:
Mozilla/5.0 (iPhone; CPU iPhone OS 13_3_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Mobile/15E148
Within the app, they append some extra data to the end like so:
Mozilla/5.0 (iPhone; CPU iPhone OS 13_3_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Mobile/15E148 [FBAN/FBIOS]
We used a similar behaviour, appending a custom value to the end of the user agent... then I just use a processing rule to check if the user agent content [***] (whatever my devs appended) and map that into a tracking parameter as being a webview from our own app (I also mapped in known Social Media apps in the same way so that I could attribute a lot of our "direct traffic" back to known social media apps.