In BI technology, there are simple words that have different meaning but are closer in concept and usage. Here I want to explain briefly two of them which are vital for every BI developers:
This is when you create a data column in a report that is enable for drill to somewhere on that report so this expands the next level of details on the same report. Most of the times the grouped fields.
This is when you create a data column in a report that is enabled for drill to another report which would suppose shows a lower level of details resides on the somewhere, outside.
More information for AX developers:
Microsoft Dynamics AX, integrated with SSRS, means that it is using the drill-through actions to open a detailed info from Enterpriseportal or open a AX form inside a report using URL action method.
In a favor of DrillThroughCommonHelper, now you can generate a required url string for SSRS URL action in order to open a form or EP directly from report.