Hi @tamara
If your client wants to get multiple websites under the same domain this is completely possible.
Indeed, if for example your client has his “main website” and a “support” website, you can with the subdomain have your main website www.example.com and your other site dedicated to support on support.example.com
So 1 domain but multiple website 
exemple: shop.exemple.com or support.exemple.com or event.exemple.com …
You could have your main site with tabs, and in one of the tabs you can integrate for example the support link. This is what Wix does in particular with support.wix.com which is different from Wix.com . So
To do this, however, you will have to purchase additional Wix hosting for each subdomain and then connect each subdomain according to this procedure
Denis