Here is the feature breakdown for that specific historical build.
If you were downloading the Beta DMG alongside macOS Sierra Beta 5, it would have been (announced at WWDC 2017). Key features included: Macos Sierra 10 12 6 Beta 5 Dmg Xcode Beta Dmg
If you come across a file named MacOS_Sierra_10.12.6_Beta_5.dmg (usually ~4.8 GB to 5.2 GB), here is exactly what you are looking at. Here is the feature breakdown for that specific
Rename stable Xcode.app to Xcode-stable.app first. Macos Sierra 10 12 6 Beta 5 Dmg Xcode Beta Dmg