Every master-report and sub-report you lay prints its sections on a position relative to its original location on paper. For a master-report, the location is always the upper left corner of the first page (x=0, y=0). Therefore all sections of that report will be printed on the left edge of the paper (x=0).
When you add sub-reports to a report, that sub-report can be located on any x-position on the paper. For banded sub-reports, usually that position is the same left-edge as the parent-report's location.
Inline sub-reports can be placed more freely on reports. The sub-report's left edge corresponds with the sub-report element's x- and y- position within its parent report. They can be at a position that is different from their parent report's left-edge position. When you add an new inner sub-report to such this sub-report, the inner sub-report's effective position is the offset of this report in its parent sub-report and all of their offsets within their respective parents.
The dark-grey area on the left hand side of your sub-report is not usable for elements contained in your sub-report. If you want to place elements there, you will have to re-position your sub-report within its parent report.
If you place elements beyond the right hand edge of your sub-report, the elements may not print fully when you leave the physical paper's area.
This now shows the sub-report at exactly the position where it will be shown on paper and allows you to easier align elements contained within different sub-reports.