Each domain (website) you operate is considered a separate website for the purposes of determining your "Domain" quota provided in your plan. For example, say your main website is myfirstshop.com. If you want to add a personal blog to host on the same account/hosting package, such as yourblog.com, some plans will allow you to do so without forcing you to purchase a separate hosting package. 

You will still need to pay for the domain registration of the additional websites you want to host (the myPolur portal will let you do this quickly and easily), but the hosting plan can be shared among multiple websites/domains you have. 

You can also decide to create a domain alias, for example, you may want my1stshop.com to redirect to myfirstshop.com; this is again considered an additional "Website" for the purposes of your plan specification.