Hi @Ivana-Tosheva, thanks for the good question.
In general, only the root domain like mysite.com and www.mysite.com will be effected.
If you have other subdomains like app.mysite.com, if you have an A Record or CNAME record pointed to some other server, that will remain unaffected by the A Record or CNAME record set for the root domain or www subdomain pointing at Webflow.
Now, in some cases, if you had your MX records pointed at a domain like mail.mysite.com, those MX records would need to be updated to point towards the server IP or url to the mailserver that hosts your email, for example if using Google Workspace, you would point your MX records away from mail.mysite.com to the Google Workspace MX records.
Each service that hosts your mail should have their own MX records that can be used.
If the current A Record for the domain is pointing mysite.com to another hosting, that need to be removed and replaced with the A Record pointing at Webflow.
If your domain provider allows, you may also add a CNAME/ALIAS record to point the Root domain like mysite.com at Webflow server proxy-ssl.webflow.com. In this case, you could also setup an additional A Record for mysite.com to point at some other service IP.
If you have specific concerns about whether a specific domain will be affected but do not want to share that publicly here in the community, feel free to contact to Webflow support on the contact page, our team is happy to help check the dns records that are currently setup and share that with you privately.
Contact the support team here: Webflow Customer Support | Webflow University
I hope this helps