What is a subdomain?
First of all, a domain is a string of a name that defines a website and is formed by following a specific procedure outlined in the Domain Names System (DNS).
For anything communicated through the Internet, be it a website, a server computer or anything else, there is an Internet protocol that defines and makes it distinctive. Such is a domain name, for example, yourdomain.com
Most domains have subdomains, for example www.yourdomain.com - secondary levels (or sublevel) of the subdomains. For instance, if your domain name of company website - mydomain.com, and support page will be located on direct link, you will get support.mydomain.com as subdomain of mydomain.com.
What is a security assessment penetration test?
A live test of the effectiveness of security defenses through mimicking the actions of real-life attackers.
Pentest can be perform by few methodology such as Information Systems Security Assessment Framework (ISSAF), Open Source Security Testing Methodology Manual (OSSTMM), OWASP Testing Guide.
The first step on each methodology is finding a organization subdomains.