The mDNSResponder project has three umbrella targets:
Build Some
Build More
Build All

The “Build Some” target builds:
mDNSResponder
mDNSResponderHelper
dns-sd tool
dnsctl
mDNSNetMonitor
dns_services
BonjourEvents

The “Build More” target builds all of “Build Some”, plus
dnsextd
ddnswriteconfig
PreferencePane

The “Build All” target builds all of “Build More” (including “Build Some”) plus
SystemLibraries

Now, the explanation for this three-layer hierarchy:

The “Build More” is the *first* target in the Xcode project.
This is what B&I builds by default.
Why not “Build All”? Because SystemLibraries is special.
B&I builds the project in two passes. B&I builds SystemLibraries first, to
make LibSystem. Then B&I builds builds the rest. The default target needs to
be everything *except* SystemLibraries — i.e. the “Build More” target.

The “Build Some” target builds the subset of “Build More”
that is useful in routine day-to-day development.
Since dnsextd, ddnswriteconfig and the PreferencePane aren’t usually
what we’re working on, it’s quicker not to rebuild them every time.

The “Build All” target is the final one. It builds absolutely everything.
B&I never builds this (they do the two-pass build) but it’s there so that
after making changes, but before checking them in, we can quickly and
easily do a “Build All” and make sure our changes haven’t broken anything.