It is often useful to execute a group only when the values of the group's tags have changed, or when a particular tag within the group has changed. In either case, the transaction group will execute every time the value or values change.
To execute only when the tag values change
In the Trigger tab, select at the very top the Only evaluate when values have changed checkbox. Now the group executes only when all the tags change. But you may want to execute the group when only one tag changes.
To execute when only one tag changes, from the Tags to watch for change dropdown, select Custom, click on Select tags, select the tag from the pop-up window, and click OK. You can select more than one tag at a time in order to monitor more than one tag for value changes.
From the Trigger on item dropdown, select the appropriate tag to execute the tag on a trigger.