The parameter is documented in [1] and is just used when the NaviServer is submitting client request to some other server via https. It is just for use in situations, where some other server sends different certificates depending on the hostname provided via SNI. The background of SNI is described in [2]... that is all quite simple and requires no complex setup.
If you are interested in serving multiple domain names via NaviServer, the probably simplest approach is to use multi-domain certificates (SAN certificates, using the Subject Alternative Name field), e.g. via lets-encrypt [3]. This is, what we are using e.g. on openacs.org.
hope this helps.
-g
[1] https://naviserver.sourceforge.io/n/naviserver/files/ns_http.html
[2] https://en.wikipedia.org/wiki/Server_Name_Indication
[3] https://bitbucket.org/naviserver/letsencrypt/src/default/
