Multiprotocol BGP (MP-BGP) Configuration

Rene, i have a question, on your 1st config on IPv6 adjacency & IPv6 prefixes, you said that
you disabled the ipv4 address family, so you disabled that by typing the command “no neighbor activate” under the address family IPv4?
so by default, the IPv4 was activated?
what will happened if you only typed…

R1(config)#router bgp 1
R1(config-router)#neighbor 2001:db8:0:12::2 remote-as 2
R1(config-router)#address-family ipv6
R1(config-router-af)#neighbor 2001:db8:0:12::2 activate
R1(config-router-af)#network 2001:db8::1/128

this is the 1st time i encounter this command, especially the address family, what does it really do? why do we need to enable or disable this one? as per your explanation, it only specifies “address family”. How the address family affects the bgp?

thanks! sorry for many questions.