In-Portal installation is divided into several steps. Because of In-Portal is module-based, then each of the modules can have it's own specific installation php code. Each of In-Portal modules can have it's "constants.php", where module developers can define module-specific constants to be used throughout the module.
While In-Portal isn't completely installed, then unit config files of all modules are included, but not module "constants.php" files. This isn't a bad thing to to, but isn't a good one either. One side effect of this is that, when we are using module-specific constants in it's unit config files, then we will get php notices about each used constant, while module is being installed.
Unit config files of all not-yet installed modules are included because they can be used in module-specific "install.php" file. Since we don't know are they used or not, then we include them in any case.
File "constants.php" of each module are only included for installed modules and this raises such a problem. Because of such file contains a constant definition, then we can't include same file twice, so I propose, that we also include "constants.php" files from all modules during installation.
This is hard piece of knowledge to understand, since developers usually don't care about how installation system of In-Portal works until their module suddenly stops installing correctly :)