Subscriber Managment on MX:minimal required dynamic-profile options

I'm going through Juniper's Day One: Subscriber Managment book and noticing some things explained here are somehow contradictional to real-world configurations.

For example, that's output from the book for dynamic-vlan (service vlan model):

[edit dynamic-profiles DYNINTF-SVLAN-DHCP-INET]
admin@SOUTHPARK# show
interfaces {“$junos-interface-ifd-name” {
 unit “$junos-interface-unit” {
 demux-source inet;
 proxy-arp restricted;
 vlan-id “$junos-vlan-id”;
 family inet {
 unnumbered-address lo0.0 preferred-source-address 12.1.1.1;
 }
 }
 }
}

(excuse me for formatting, it's copied from PDF)

That's what book sys about this config:

"The key difference between this configuration and the one in Chapter 2, of course, is the inclusion of the demux-source inet statement. This instructs Junos to use the IPv4 address of the packet to create a unique subscriber session anchor."

 However, this explanation seems really weird cause all examples I found in documentation states "demux-source" is used on the IP demux interface and not on the dynamic vlan interface. 

 I also have a working configuration without that line:

root# show dynamic-profiles VLAN-demux                        
routing-instances {
    "$junos-routing-instance" {
        interface "$junos-interface-name";
    }
}
interfaces {
    demux0 {
        unit "$junos-interface-unit" {
            vlan-id "$junos-vlan-id";
            family inet {
                unnumbered-address "$junos-loopback-interface";
            }
        }
    }
}

Another thing that's not clear to me is underlying-interface. That's config from the book:

DYNSUB-SVLAN-IPDEMUX {
 interfaces {
 demux0 {
 unit “$junos-interface-unit” {
 demux-options {
 underlying-interface “$junos-underlying-interface”;
 }
 family inet {
 demux-source {
 $junos-subscriber-ip-address;
 }
 }
 }
 }
 }
}

Documentation doesn't state underlying-interface parameter is optional..which, I suppose, means it's required.

Yet here's my config that works perfectly without it:

root# show dynamic-profiles DHCP 
routing-instances {
    "$junos-routing-instance" {
        interface "$junos-interface-name";
    }
}
interfaces {
    demux0 {
        unit "$junos-interface-unit" {
            family inet {
                demux-source {
                    $junos-subscriber-ip-address;
                }
                unnumbered-address "$junos-loopback-interface";
            }
        }
    }
}

 What's the purpose of this line then?