BTAD_InstallDir
BTAD_InstallDir contains the installation path of the BizTalk application. This is only available when the action is Create or Update, and BTAD_HostClass is BizTalkHostInstance.
BTAD_InstallDir is set to the TARGETDIR property of the Windows Installer package. The default value is %ProgramFiles%\Generated by BizTalk\<application name>.
Pre- and Post-processing Script Environment Variables
How Environment Variables Indicate Deployment State
How to Install a BizTalk Application