For trackers with embedded tables like test cases or test sets, there is an optional embedded Table selector.

This allows to map input items not to main items of the target tracker, but to rows in the selected embedded table of a main/parent target tracker item.

From another example ReqIF import:

Mapping items to an embedded tracker item table (For example, Test CasesTest Steps) only works if a mapping for the main tracker items has also been created. (for example, test cases) and there is a specification that establishes the parent/child relation between the main tracker items (for example, test cases) and their respective sub items (for example, test steps) in its hierarchy tree.