I didn't understand the "validate your certificate" section

Hi, I’m new to security with certificates and having trouble to understand this section.

  • Why do I need to do this ? (I’m using my own CA)
  • In cat cert1.crt cert2.crt cert3.crt cert4.crt >> mms-ca.crt do I have to add my ca.pem in the concatenation ?
  • Can I use the same CA certificate and key for the whole process or do I need several ?