Whether set to OnBoot or OnDemand, you start it by clicking on the icon in the wbar, or clicking on the name in the menu. Nothing more.
OnBoot means the application is mounted when you start the computer, but not started until you start it.
OnDemand means the application is not mounted when you start the computer. When you start it, it is mounted, then started.
Having apps in OnBoot means there is more in ram. Having apps in OnDemand, uses less ram.
Having apps in OnDemand results in the computer starting faster. When in OnBoot, they all need to be mounted, so the computer starts slower.
In OnDemand, the apps may start slower. Less than a second slower for most apps.
I have most apps in OnDemand. The computer starts quick, and uses less ram, even though I have enough ram to run them all in OnBoot.
I have a few apps in OnBoot. Some examples:
I run the Iptables firewall in OnBoot, and start it from /opt/bootlocal.sh. This way I don't need to do anything. It just runs every time I start the computer.
I run wget in OnBoot. Wget runs from the terminal. When you want to do a download, just do it. If you run it in OnDemand, you need to start if from the menu, before doing a download using the terminal.
It is a good idea to have sound in OnBoot, so it just works when you start the computer.
I run most other apps in OnDemand.
Another thing to be aware of using OnDemand. If you want to run an application as root, you need to run it as a regular user, and close it, before running it as root. For example, if you want to run Leafpad as root to edit certain files, you need to run it as a regular user first.
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Extensions are automatically mounted. You don't need to do anything.
Applications are extensions in /tce/optional.
If you run them in OnBoot, or start them after running them in OnDemand, they are mounted in /tmp/tcloop. Files in the apps then become part of the filesystem. You can check, by running them in OnDemand, and seeing more extensions mounted when you start an app. Dependencies are also mounted in the same way.
I hope this makes it clearer.