This test will be accessible by those who register for the Halo Insider program and will include "hundreds of thousands" of players who are invited before it begins.
The new mode that will be available in this test is the Bot Arena playlist, which puts 4 players against 4 bots on Arena maps. Players will also get access to the menu and Battle Pass UI, the Academy's Weapon Drills, and new Waypoint experiences on the web, iOS, and Android.
If this technical test goes well, even more Insiders will be invited to the following previews.
The blog post said that the reason for the limiting of players is that the team "want to start at a reasonable number of participants before potentially going to a much larger player pool, but our goal is to eventually get every eligible Halo Insider a chance to go hands-on and help us achieve our goals on the road to launch."
You can find out how to sign-up for the Halo Insider program here to be in with a chance of checking the game out before its Holiday 2021 release.