Clarity on usage of service ID

[mredig]

What is the purpose of the service ID?

I had previously thought that the service id was what you used to identify your app so that your keys would be in a unique store, separate from other apps on the system. But then I thought, "what's to stop another app from just reading or changing your keychain items if they happen to know what serviceID you're using?" That just doesn't make sense that apple would allow a system like that (and I believe that's not possible/you'd need to use access groups with an app group bundle id, but would still just limit to just apps that are in that app group).

so I'm back to just being confused about its purpose. I cannot find anything in the documentation to indicate what you'd use the service id for, nor why you might want to use one at all, nor why you might use more than one? Are there any security implications in choosing one over another/how we should store the string of the serviceID... etc?

The example app just adds to my confusion as it demos the usage of multiple service IDs or none at all.

Could we get documentation on this topic? The framework is working for me, but I cannot be certain I am using it correctly or securely without these considerations. Thank you!