Along with your domain’s SSL certificate, Comodo gives you a few seemingly random files. The SSLCACertificateFile directive is used for client authentication. It’s basically a single file with PEM-encoded CA certificates concatenated together. They are concatenated by order of preference:

cat COMODORSADomainValidationSecureServerCA.crt AddTrustExternalCARoot.crt COMODORSAAddTrustCA.crt > mydomain.cer

This particular example was on an Ubuntu 14.04.1 LTS server with Apache 2.4.7.