${ROOT_TREE}

When creating a package with just a control file, the root directory is determined as the parent folder from where the control file is found. So if your control file if found under dist/WPKG/control, all the other folders under dist are taken as the content of your package.

In case of a control.info file, this is not practical because each package needs its own control file. By default, the --build command accepts two parameters. The first one is the name of the control.info file and the second a path to the package data. The second parameter is optional in which case you must indicate the path using the ROOT_TREE variable extension (the command line argument as priority meaning that we ignore the ROOT_TREE variable if defined.) The path can be absolute in which case it is used as is. If not absolute, the path is added to the path where the control.info file resides.

Remember that when building packages for a control.info file, the top directories are named after the names found in the Sub-Package field. So for example if you used runtime and doc as Sub-Package names, then the respective paths for the package contents are: ${ROOT_TREE}/runtime and ${ROOT_TREE}/doc.