A dependent request is a request that is dependent upon an existing request.
For reports with correlation breakdowns enabled within the report table, to take a closer look at an item, right-click in the cell and then click Add Dependent Request > Breakdown.
You can also add a breakdown request when you create 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.