Count Sub-Items Column
The Count Sub-Items column allows you to quickly see how many items are beneath a work item or flex item 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 items have been assigned to each resource (when your items 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.