In ControlDraw, Hierarchical Tags are tags/names for symbols that are made up from the names that appear on the diagrams for each level of the model Hierarchical. They should provide a unique name for each instance of a symbol.
For example say the first diagram has a unit named u1123 on it, with a parent link to page 2.
Page 2 has en equipment modules named em001 on it, linked to page 3
And page 3 has a valve, v01 on it
Then the Hierarchical Tags of v01 on page 3 is then
Now, Page 1 might have another unit on it, u1124
u1124.em001.v01 is another instance of v01 on page 3
So, Hierarchical Tags this is a tag that applies to an instances. They are unique to each instance provided there are no duplicate symbol names on a diagram.
But in large models this can be cumbersome and for other reasons a better (shorter) tagnames could be used. That is the reason for RealTags.