As a part of the STARTTLS form of authentication and encryption, certificates are usually exchanged. If a certificate is presented by the other side, this ${cn_issuer} macro is assigned the CN of that certificate. That value might look like this:

Juan+20Garcia        a person's name
Garcia's+20Software  a business name

See Section 21.6.2 to find how and why the value in this macro undergoes special translation.

${cn_subject} is transient. If it is defined in the configuration file or in the command line, that definition can be ignored by sendmail. Also note that a $& prefix is necessary when you reference this macro in rules (that is, use $&{cn_subject}, not ${cn_subject}).