[exim] Doubt with main configuration macros

I don't know how to read the exim4.69 configuration files. I'm using split config.
When I read main/01_exim4_config_listmacrosdefs I don't know where are defined some macros. e.g.

.ifndef CONFDIR
CONFDIR = /etc/exim4
.endif

CONFDIR where is defined?

Some macros are defined at update-exim4.conf.conf

Another example at main/02_exim4-config_options

.ifndef MAIN_ACL_CHECK_RCPT
MAIN_ACL_CHECK_RCPT = acl_check_rcpt
.endif

Thanks