Should i use a stereo or mono bus for plugin?

I want to use a plugin which is shown as 1/2 in the selector. Does this mean it has a mono input and stereo out?
Should i use a mono or stereo bus for this plugin.
I recently started using AVl mx21 and found that when i mixed up mono and stereo plugins in the same bus, ardour kept crashing
Many thanks

I believe that is correct.

You would usually use a mono bus, that way mono signals can be summed in the bus and the sum sent to the plugin. The plugin outputs would connect to a stereo bus, such as the main mix bus.

That is not expected, I have not heard that is a known bug in Ardour. That version of AV Linux has Ardour 6.9.0, correct?
Mixing mono and stereo plugins can be done, but you have to do more manual routing work to make sure all the audio connections match what you want.

It is possible that one of the plugins crashed, and due to the way plugins work a plugin crash causes the host application to crash (with a few exceptions for hosts that run plugins in separate processes, but Ardour will crash if a plugin crashes).

If you remember which plugins you used and can file a bug report describing what you did that led up to the crash it would be helpful. Especially if the plugins were all included with AV Linux I’m sure GMac would like to know if one of the included plugins is buggy.

Thanks Chris for this in depth explanation.
Yes it is Ardour 6.9.
I will look through the plugins list and see if I can work out which one gave the trouble. It was when I had just started using AVL and many of the plugins were unfamiliar to me. Maybe seeing the list will jog my memory!