Extension inx files have the tag <object-type></object-type> with possible values:

• path
• all
• and likely others

What's the impact or purpose of this?
Where can I find more documentation on this?

Maybe here? https://inkscape.org/develop/extensions/

Thanks - that lead me to this which explains that the values can be set to all or any element type:

[all|{element type}]

However, I can't find what/how this will impact the extension.

Not my cup of tea. ;-)

In my experience it does nothing.

Even in the tutorial on the new extension documentation: 

https://inkscape.gitlab.io/extensions/documentation/tutorial/my-first-effect-extension.html

You can see it has been commented out.

I have asked in chat, but I suspect it is a legacy feature, possibly from before 0.48, I did a quick search of the source code, no real reference to it ? 

I've changed it in my extensions - no effect - I would just set it to 'all' ( although I've always had it set to 'path' ) for no particular reason.

Perfect, thanks inklinea!