A cell of rank k is also called a rank-k cell or k-cell. A 0-cell is an atom, a 1-cell is a list, a 2-cell is a table, and so on. If the rank of the cells of a noun is given, then the frame is whatever is left over of the shape of the noun.
Negative numbers are also used, as in _2-cell and _1-cell; the frames of such cells have length indicated by the magnitude of the numbers. You have seen _1-cells before: they are items.
abc =. 4 3 2 $ i. 24
The noun abc can be thought of as:
a 4 3 2 frame of 0-cells
a 4 3 frame of 1-cells
a 4 frame of 2-cells
an empty frame of a 3-cell
A more general way of phrasing this is:
a rank 3 frame of 0-cells
a rank 2 frame of 1-cells
a rank 1 frame of 2-cells
a rank 0 frame of 3-cells