For example, Host/Template has items called;

• item.name[a].
• item.name[b].

In the definition of the item, you have a field named something like "value macro". The user enters their desired macro name here. When a value is received, it is inserted both to history, and to it's assigned "value macro".

• host:item.name[b] = VALUE2 => {%VALUEMACRO}
  
  So item.name[b]'s value now becomes available in {%VALUEMACRO}

  . I have no idea for scope but it would makes sense that it would be in the host scope as the item is assigned to the host.

The use case is being able to say in a trigger;

• Trigger: {host:item.name[a].last(#1)}

  =

  {%VALUEMACRO}

I believe this addresses the issue where you cannot compare two items values in a trigger, particularly if they are a string. That problem would be demonstrated as;

• Trigger: {host:item.name[a].last(#1)}

  =

  {host:item.name[n].last(#1)}