What does option ipv6 in PPPoE interface do?

Installing and Using OpenWrt
1

Hi guys. I've read the documentation, still I do not understand what the option ipv6 does.

With the most basic form of configuration:

config interface 'wan1'
	option device 'eth1'
	option proto 'pppoe'
	option username '<pppoe_username>'
	option password '<pppoe_password>'
	option ipv6 'auto' # 'auto' is the default value

OpenWrt automatically spawns a wan1_6 interface of type Virtual dynamic interface (DHCPv6 client), which gets an IPv6 address, and a Delegated Prefix from the ISP.

After that I try to change the configuration into:

config interface 'wan1'
	option device 'eth1'
	option proto 'pppoe'
	option username '<pppoe_username>'
	option password '<pppoe_password>'
	option ipv6 '0' # it doesn't matter if the value is '0' or '1'

config interface 'wan1_6'
	option device '@wan1'
	option proto 'dhcpv6'

I see the result is almost the same, except that the wan1_6 interface protocol is now Alias Interface (DHCPv6 client).

My questions are:

  1. Should I explicitly create an alias interface, or let the IPv6 interface be spawned automatically? Which one is the best practice?
  2. If I leave option ipv6 at auto and let the wan1_6 be spawned automatically, do I need to add wan1_6 to firewall wan zone? Because such interface does not exist in /etc/config/network.
  3. What is the difference between value 0 and 1 of option ipv6 if I manually configure an alias interface with option proto 'dhcpv6' as above?