In accordance with the requirements of the CA / Browser Forum starting 8 September 2017, SSL certificate issuers will verify the CAA records in the DNS records of clients applying for SSL certificates.
If you enter a specific CA name, only the Certificate Authorities will be able to issue the certificate.
Starting 1st of August 2016, Comodo and DomenySSL will no longer offer SGC variants of certificates. As your account has a valid SGC certificate which will be up for renewal in the future, the company has prepared a list of recommended alternatives.
Thawte is only one of the few vendors outside of the United States. As the main competitor of American vendors quickly gained a 40 % share of the market SSL certificates.