What it is
The App Portal is where the applications come from. IGEL OS 12’s base image deliberately contains none, so the Citrix client, the Omnissa Horizon client, the Azure Virtual Desktop client and the browsers are all selected in the portal and pushed to devices or device groups from there.
It is part of the IGEL cloud services rather than something you install.
Why the split matters
Applications are delivered as containers, managed apart from the operating system. So a client update is a package update, not an OS release, and the two stop being scheduled against each other.
That is the actual argument for IGEL OS 12 over 11 in an estate where the VDI client version is dictated by someone else’s upgrade calendar.
What to watch
An estate with no route to the internet does not reach the portal. IGEL’s answer is the distributed app repository, where devices pull packages from a local server instead, and that is an Enterprise-licence feature. There is also peer update, where a device pulls from another device on the same network, and that one is in every edition.
Which of the two you need is an air-gap question, and it belongs in the design.