If the flex container has a percentage based cross size and we can get the available cross size from its containing block, then it is considered to have a definite cross size. This is used when computing the transferred size suggestion: 

If the item has an intrinsic aspect ratio and its computed cross size property is definite, then the transferred size suggestion is that size (clamped by its min and max cross size properties if they are definite), converted through the aspect ratio. It is otherwise undefined.