A subdomain is a subdivision of a domain. Therefore, you must have registered at least one domain before you can create a subdomain. The number of subdomains that can be created depends on the type package you have.
You can create multiple subdomains of your domain to meet your needs. For example, websites can be divided up systematically using subdomains. They are often used to highlight certain services or products and make them more accessible. You can also assign email accounts to each individual subdomain - e.g. for different departments or branches.
ionos.co.uk is a domain, while the subdomain login.ionos.co.uk refers directly to the 1&1 IONOS login page.
All the letters from A-Z, digits from 0-9, and hyphens (-) are allowed for a subdomain name (as long as 2 or more hyphens are not consecutive). The name must start and end with a letter or a number, and spaces are not allowed.
Other than this, you can freely assign any name for a subdomain. For example, you may want to distinguish between different parts of your company with subdomains, such as "sales.example.co.uk", "mobile.example.co.uk", or "london.example.co.uk".
Using the example above, you could then create email addresses using the subdomains, such as "firstname.lastname@example.org" or redirect the subdomains to a specific page of your website or another website entirely.