"Ship Groups" are what the text is referencing. If you open X amount of [santa container] without hitting a Ship Group, the next of that [santa container] will draw from a Ship Group.
If all the options in one of the Ship Groups are exhausted, the other Ship Groups gain the % chance. The overall chance of striking the Ship Group remains the same.
If you have received ALL of the ships on offer in all of the Ship Groups, you will receive steel instead if a Ship Group is pulled.