[exim] $primary_hostname and $smtp_active_hostname

I'm in the midst of modifying my config file from a 4.34 system to add
features in 4.50 and I noticed the addition of smtp_active_hostname and
$smtp_active_hostname. The documentation (section 14) says,

"The value of $primary_hostname is also used by default in some SMTP
response messages from an Exim server. This can be changed dynamically by
setting smtp_active_hostname."

I interpret this to mean that $primary_hostname is set to the value of
smtp_active_hostname after expansion, so essentially both $primary_hostname
and $smtp_active_hostname both have the same value. If so, what was the
point to making the default smtp_banner use $smtp_active_hostname rather
than $primary_hostname if both variables are the same?

I didn't think to ask any of this until I saw the description for
message_id_header_domain which said that it defaults to the "primary host
name". If both variables are the same then that's dandy, but wouldn't
having $primary_hostname always set to the value in primary_hostname be of
benefit to some people? If it was, then there'd be a ton of places where
you would no longer want to use $primary_hostname, but rather
$smtp_active_hostname - including this one here.

If I'm mistaken and the two variables DO NOT share the same value, then for
example is message_id_header_domain defaulting to $smtp_active_hostname or
$primary_hostname?

I'd test all this but I have not completed configuration changes and
installations of some more software to make everything work. I could be
making a big deal out of nothing of course - wouldn't be the first time :P
I'm just trying to figure out if I should be changing everything that uses
$primary_hostname to $smtp_active_hostname, or if there's no need since both
variables are the same (which then begs to ask, why invent
$smtp_active_hostname).

Eli.