CA Bundle file contains root and intermediate certificates. These certificates build the chain of trust for your domain certificate.
The bundle files differ by the validation level of your certificate and the key type in the CSR you used to activate your certificate.
The CA Bundle file can be downloaded from your SSLs.com account. The CA bundle file has .ca-bundle extension.
Alternatively, feel free to use the CA bundle files from this article.
If your certificate was issued after Jan 2019, please download current CA-bundle. Else, download the corresponding old bundle from Comodo.