A Disposable VM (DispVM) is a lightweight VM that can be created quickly and will disappear when closed.
Disposable VMs are usually created in order to host a single application, like a viewer, editor, or web browser.
Changes made to a file opened in a Disposable VM are passed back to the originating VM.
This means that you can safely work with untrusted files without risk of compromising your other VMs.
DispVMs can be created either directly from Dom0 or from within AppVMs.
Once a DispVM has been created it will appear in Qubes VM Manager with the name "dispX".
See [this article](https://blog.invisiblethings.org/2010/06/01/disposable-vms.html) for more on why one would want to use a Disposable VM.
Disposable VMs and Networking
-----------------------------
NetVM and firewall rules for Disposable VMs can be set as they can for a normal VM.
By default a DispVM will inherit the NetVM and firewall settings of the VM from which it is launched.
Thus if an AppVM uses sys-net as its NetVM, any DispVM launched from this AppVM will also have sys-net as its NetVM.
You can change this behaviour for individual VMs: in Qubes VM Manager open VM Settings for the VM in question and go to the "Advanced" tab.
Here you can edit the "NetVM for DispVM" setting to change the NetVM of any DispVM launched from that VM.
A Disposable VM launched from the Start Menu inherits the NetVM of the [DVM Template](https://www.qubes-os.org/doc/glossary/#dvm-template).
By default the DMV template is called `fedora-XX-dvm` (where `XX` is the Fedora version of the default TemplateVM).
As an "internal" VM it is hidden in Qubes VM Manager, but can be shown by selecting "Show/Hide internal VMs".
Note that changing the "NetVM for DispVM" setting for the DVM Template does *not* affect the NetVM of DispVMs launched from the Start Menu; only changing the DVM Template's own NetVM does.
Opening a file in a Disposable VM via GUI
-----------------------------------------
In an AppVM's file manager, right click on the file you wish to open in a Disposable VM, then choose "Open in Disposable VM".
Wait a few seconds and the default application for this file type should appear displaying the file content.
This app is running in its own dedicated VM -- a Disposable VM created for the purpose of viewing or editing this very file.
Once you close the viewing application the whole Disposable VM will be destroyed.
If you have edited the file and saved the changes, the changed file will be saved back to the original AppVM, overwriting the original.