What's the purpose of the TXT record in DNS configuration for a subdomain?

When connecting a subdomain (testsubdomain.example.com) to Webflow, in a case where the main domain (example.com) is not meant to be connected, Webflow asks to add 2 records:

Type Name Value
CNAME testsubdomain proxy-ssl.webflow.com
TXT @ proxy-ssl.webflow.com

What’s the purpose of the TXT record? why is it added to the main host and not subdomain? Will deployment work without it?