Apple - Dual Boot Created a New Volume

macOS Catalina has a new feature to further isolate the OS from data. In the APFS container it is installed within, an APFS volume for the OS is created alongside one for your data. The former is mounted read-only within Catalina, and the latter has the Data postfix containing your apps and data.

https://developer.apple.com/videos/play/wwdc2019/103/

Within Catalina, Disk Utility shows these volumes with a Finder icon and home icon to distinguish them. This does not appear in Mojave, showing as normal volumes. Keep in mind the release notes for the beta of Catalina, specifically dual booting and Spotlight indexing, which will heavily confuse Mojave with the cross-volume linking of various folders.