AFAIK, this refers to several cable trays stacked one above the other, as might be used when space does not permit of wider tray or multiple trays side by side.
The rating for the bottom tray is the same as for a single tray, since the cables on the bottom tray do not “know” that other trays containing other cables are above them.
The rating for the upper trays is slightly reduced because cables thereon are subjected to warmed air that rises from the lower tray(s)
All relevant derating factors are applied. So if derating to 0.88 is required for number of cables, and derating to 0.93 is required for an upper tray, then the final figure is 0.88 multiplied by 0.93.
