TwistedPlayground/android_build — comment on commit 0eb05d656aca637193d036287671471e3d7e4dc6
File:  line 0
By TwistedUmbrella:

"Unlike backup tool, which uses the cm prefix to an existing lunch combo
variable, this flag can apply to devices not currently supported by CM.
The supported devices are manufactured by HTC, so instead of verifying
a cm prefix, the htc_ component is used to trigger the append."

^ it uses the htc_ part of your TARGET_PRODUCT choice to call it.