Hi Neil and welcome to the forum.
What you have to put in [class="???"]
depends on the window you want to match. Therefore you will need to have to find a unique feature that distinguishes your window from every other window.
You can use a little linux/unix tool called xprop
to get the X properties of any window. It is probably already installed on your distro. Just type xprop
in a terminal to test it.
Now you will get some information on your window such as:
...
WM_CLASS(STRING) = "urxvt", "URxvt"
...
When clicked on a simple urxvt terminal for example. (Your output includes much more information and of course the class name of the window you clicked on).
Now the interesting part:
In my example "urxvt"
is the instance
name and "URxvt"
is the class
name that you can use to match any window.
Attention the names are both case sensitive.
Most of the time a class and maybe an instance should suffice to tell your window apart from any other. But sometimes you will need more sophisticated queries. The best source for all possibilities is the i3 documentation or some real life samples from my config.