Parent apps can be useful if you have a complex base program and a lot of customers that should have their own database but not an instance of a sourcecode.
For example you have a timetracking app for different companies and each of them does things a bitt different. But you don’t want to copy the base sourcecode for every company all the time.
In a child/parent app, bpfw looks for a file in the child folder first, then in the parent app and finally in the bpfw folder. So you can place all your common files here.
To set this up:
Create an app the usual way
set in the config.inc.php of the app. This will be the parent app:
defined("TEMPLATE_THEME") || define('TEMPLATE_THEME', true);
Now create another app in inside the config:
Thats it, You could delete the parent database now since it is no longer used.