While adding custom domain, why we need to add two sub-domain?

[Himanshu Kumar]

I am using Jira Software (Cloud). By default they have provided the site URL with atlassian domain name i.e. https://xyz.atlassian.net but I don't want to use their domain name in my Jira site URL. When I am trying to add custom domain then they are asking to provide two sub-domain name and it's mandatory, we can't skip i.e. https://abc.pqr.stardom.co.in. Atlassian team saying that it's because security reason we need two sub-domain. It means whatever the site URL provided by the Atlassian, it's not secure. Please suggest me what should I do?