What?
As the name suggests, all members of an alliance have interlining set up automatically if desired. Consequently, existing agreements between individual members are superseded/replaced and any rules introduced by Advanced Interlining (like booking code mappings) will apply across the whole alliance.
Since this transition might be non-trivial (code mappings might not make sense, a new member’s PSS might not support required features, …), this could be a semi-automatic process that a new member needs to complete before becoming a full member of an alliance.
Why?
This has been a long-requested feature. In fact, many new players get confused because this isn’t the current behavior today already. Especially with the new complexity and possibilities introduced by the Distribution System, such a feature would add even more value.
When?
Almost certainly only makes sense once Advanced Interlining has been implemented and it is clear what the requirements are to make automatic interlining work with the new system.