Hi

First, I installed JMS on /home/usr/domains/xxx.com/subdomains/master/public_html

I then set up a domain on /home/usr/domains/yyy.com/public_html

When I set up a template to install a slave site on yyy.com, I use the deployment

/home/usr/domains/{site_url}/public_html

but I get the following error:[/home/usr/domains/[b]master.xxx.com[/b]/public_html] does not exists.

Why is {site_url} using the master.xxx.com when deploying the site instead of yyy.com? Is my configuration wrong or should I be using a different keyword?

The deploy folder is wrong.
You can not use the {site_url} that is a URL and not a directory name.

If you had use the {site_id} to create the domain name (ie domain = {site_id}.com), you could reused it into the deploy folder.

Read the yellow light that provide the description of each keywords.

The {site_url} is the URL that you have currently in your browser.
So probably master.xxx.com that is your current website (the current master URL).