Count Sub-Items Column
The Count Sub-Items column allows you to quickly see how many items are beneath an issue or folder within a structure.

In the example above, we can see that there are only 3 sub-items beneath epic GREEN-40, but 57 sub-items are in the GreenCycle group.
Use Cases
The Count Sub-Items column can be used to identify:
How many items are contained within a folder or other grouping
How many issues have been assigned to each resource (when your issues are grouped by Assignee)
How many tasks are necessary to complete each story, epic, or initiative
Depending on how you've grouped your items, you may not want to count all sub-items. Instead, it may be more useful to just count the bottom level. In this case, you could use the Count Leaves column.