A dependent request is a request that is dependent upon an existing request.
For reports with correlation breakdowns enabled within the report table, you can take a closer look at an item by right-clicking in the cell and clicking Add Dependent Request > Breakdown.
You can also add breakdown request while creating the original request. See Create a Data Request.
Adobe Report Builder enforces request dependencies only within the same worksheet, not across worksheets. Restricting to dependencies within a single worksheet ensures timeliness of execution.
See Breakdown Reports.