Also written as sub-domain, this is the part that is before the domain name. In the case of this website, terms.fsp.fyi is the subdomain of the for this site. As for any restrictions on subdomains, this will typically be set by the host, and almost always not case sensitive. When possible, I will find a hosting company that will give you unlimited subdomains for your domain. This is so there aren’t any restrictions if your domain grows.