Hi
I want to copy one item value to another. The trivial way is to use rule. I was trying to use group for that but it seems not to work.
Number X (Y)
Group:Number Y
Any idea why? Is there a better way?
Thx
You have to use a Rule. There is no other way.
Thx for the quick reply 
Why group will not work?
Because that isn’t what Groups do.
Groups are a way to aggregate a bunch of Items together to provide a summary of all the Item’s states, command all the Items as one thing, or to reference and process a Group of Items in Rules together.
Groups do not copy the value of one Item to another.
That being said, what do you mean by “seems not to work”? What specifically are you trying to achieve and what specifically are you seeing?
It might be the case with Group:Number that you must provide the aggregation function as well if what you are seeing is that Y remains uninitialized (i.e. has a “-” for its value on the sitemap).