Virtual directories can be used to run multiple web applications on a single domain.  This is similar to subdomains, but in the case of virtual directories, the directory is access using the folder path instead of the sudomain name.  For example if you have a virtual directory in the domain test.com named virdir, the virtual directory is accessed via test.com/virdir.  If you setup the virdir directory as the subdomain named virdir.test.com, it is access via virdir.test.com/.  

See How to Determine Which Version of Plesk Control Panel is Installed on a Server to know which set of directions to follow.

1. Log into your Control Panel.
2. In the Subscriptions tab, Click on your domain name.
3. Click on the Websites & Domains tab.
4. Under Web Hosting Access, click Show Advanced Operations. 
5. Click on the Virtual Directories icon.
6. Click on Create Virtual Directory.
7. Configure virtual directory settings as needed and click OK.

1. Log into your Control Panel.
2. Click on the Home tab.
3. Click on Domains.
4. Click on your domain name.
5. Under the Files section, click on the Virtual Directories icon.
6. Click on Create Virtual Directory.
7. Configure virtual directory settings as needed and Click OK.