This directory is dedicated to store external modules. To use it, just copy here the directory of the module into this directory.
Note: On linux or MAC systems, it is better to unzip/store the external module directory into a different place than this directory and just adding a symbolic link here to the htdocs directory of the module.
For example on Linux OS: Get the module from the command
mkdir ~/git; cd ~/git
git clone https://git.framasoft.org/p/newmodule/newmodule.git
Then create the symbolic link
ln -fs ~/git/newmodule/htdocs /path_to_dolibarr/htdocs/custom/newmodule
WARNING !!!
Check also that the /custom directory is active by adding into dolibarr conf/conf.php file the following
two lines, so dolibarr will also scan /custom directory to find external external modules:
$dolibarr_main_url_root_alt='/custom';
$dolibarr_main_document_root_alt='/path_to_dolibarr/htdocs/custom/';