Here's a full guide on the "Manifest" variable.
It determines the branch and the manifest link of the recovery you wanna build.
Here's the syntax and an example.
<manifest link> <branch>
git://github.com/SKYHAWK-Recovery-Project/platform_manifest_twrp_omni.git -b android-9.0
So if you have any problems, read this.
Thank you.
By Jamie