The Custom Action Target field is not applicable to InstallScript Custom Actions.

This field should contain an execution parameter that will vary, depending on the basic type of custom action. For example, this field may contain the following depending on the custom action.

Target | Custom Action
Entry point (required) | Calling a DLL.
Executable name with arguments (required) | Calling an existing executable.
Command line arguments (optional) | Calling an executable just installed.
Target file name (required) | Creating a file from custom data.
NULL | Executing script code.