What is the difference between "config system pppoe-interface" and "set mode pppoe"?

Question

Hello,

I´d had to configure PPPoE for use with an external VDSL-Modem on several FortiGate. The data in the PPPoE tunnel have to be tagged in VLAN 7.

I have the following questions on that:

1. What in general is the difference between "config system pppoe-interface" and "set mode pppoe"?

2. How can I set VLAN-tagging for the interface which is configured with "config system pppoe-interface"?

3. Any hints why the first config ("set mode pppoe") did not run on the FG-61F?

On the first Fortigate 100F with FortiOS 7.4.4 it worked fine with the following configuration.

config system interface
edit "ppp0"
set vdom "root"
set mode pppoe
set allowaccess ping https ssh http speed-test
set alias "VDSL-Bridge VID7"
set role wan
set snmp-index 18
config ipv6
set ip6-mode pppoe
end
set username "xxxxxxx@yyyyy.zzz"
set password ENC xxxxxxxxxxx
set interface "wan1"
set vlanid 7
next
end

But on another FG-61F with FortiOS 7.4.3 and the same provider the above settings did not work. I had to use the following config.

config system interface
edit "VDSL"
set vdom "root"
set mode pppoe
set allowaccess ping https ssh
set type tunnel
set role wan
set snmp-index 20
set interface "wan1"
next
end