Subdomains can be helpful in creating dev sites or mobile or forum installations eg. dev.yourdomain.tld or forum.yourdomain.tld, separate from your main site.
Cpanel makes this process simple and we will help you around this, please follow the steps to create subdomains:
Step 1. Login to cPanel from client area
Step 2. Under Domains section selects 'Subdomains'
Step 3. Fill the required fields.
Step 4. Click 'Create'.