Direct dial is a feature that allows a DN to be reached with a single digit, usually 0. So, any set in the system can press an Intercom button, dial 0 and have their call ring the direct dial DN.
Extra dial is an alternate direct dial that can be turned on and off with a feature code. When Extra dial is turned on, any set in the system can press an Intercom button, dial 0 and have their call ring both the direct dial DN and the extra dial DN. Extra dial is part of the ringing service. As such, it can be turned on only when a ringing service is active.
A set can transfer calls using the Direct dial digit: simply press the TRANSFER softkey and dial 0. However, I don't think that an AA can transfer calls using the Direct dial digit. Direct dial is not an actual DN, it is more of a system feature.
If you are only interested in calls transferred by the AA, you could achieve a behavior similar to Call Forward with sequential hunt groups and the Do Not Disturb feature:
[ol 1]
[li]Set up the AA to transfer the calls to a hunt group DN instead of transferring them to DN 11 directly.[/li]
[li]Add DN 11 as the first member of the hunt group.[/li]
[li]Add the forward DN as the second member of the hunt group.[/li]
[li]Set the Distribution Mode to "Sequential".[/li]
[li]Set the Hunt Delay so it gives the person at DN 11 a chance to answer.[/li]
[/ol]
Now, when the AA transfers a call, it will ring the first member, which is DN 11. To forward the transferred calls, turn on Do Not Disturb. The transferred calls will ring the second member.
As you probably know, a hunt group only allows one call at a time per member. It means that if the first member is on a transferred call and another call is transferred, it will ring the second member, without the first member being alerted. You can work around this issue by using more hunt groups. The following setup would allow 2 simultaneous transferred calls:
[ol 1]
[li]Set up the AA to transfer the calls to a hunt group DN (let's call it HG 1) instead of transferring them to DN 11 directly.[/li]
[li]Add DN 11 as the sole and only member of HG 1.[/li]
[li]Set the Distribution Mode to "Sequential".[/li]
[li]Set the Overflow to another hunt group DN (let's call it HG 2).[/li]
[li]Set the Busy routing option to "Overflow".[/li]
[li]Add DN 11 as the sole and only member of HG 2.[/li]
[li]Set the Distribution Mode to "Sequential".[/li]
[li]Set the Overflow to another hunt group DN (let's call it HG 3).[/li]
[li]Set the Busy routing option to "Overflow".[/li]
[li]Add the forward DN as the sole and only member of HG 3.[/li]
[li]Set the Distribution Mode to "Sequential".[/li]
[li]Set the Overflow to another hunt group DN (let's call it HG 4).[/li]
[li]Set the Busy routing option to "Overflow".[/li]
[li]Add the forward DN as the sole and only member of HG 4.[/li]
[li]Set the Distribution Mode to "Sequential".[/li]
[/ol]
In other words, there are 4 hunt groups: 2 that have DN 11 as their only member, and 2 that have the forward DN as their only member. A transferred call would be routed from one hunt group to another until it can ring a member.