The "X chamber" construction is the most simple way to enclose the filling into chambers: the inner and outer fabric are just quilted together, forming tubular chambers that are filled with loft material, as it is known from most down jackets. It is often also named "sen-through", e.g. in this article. In contrary the other chamber constructions are built in a way that the chambers have some overlap of loft material to prevent thermal bridges. This does also mean that inner and outer fabric do not share common seams.
The drawback of the "X" construction compared to the other construction types ("H", "V" etc.) is that the sleeping bag has no insulating material at the seams which reduces the thermal insulation. The advantage is that it is lighter than the other constructions as there is no additional fabric needed besides the inner and outer shells to form three dimensional chambers. This is OK for sleeping bags for not too cold temperatures but is not so good when going to lower temperatures where there would be significant heat loss through these uninsulated seams.