[css-flexbox] what's a "definite main/cross size" exactly?

Hi there!

In https://drafts.csswg.org/css-flexbox/#definite-sizes, both points
require a definite size of the flexbox. This is of course easy when a 123px
size is given, or a resolvable percentage size.

But for an auto size, it is a bit less clear. Given that percentages of
children of abspos elements can get resolved, should we consider an abspos
flexbox to have a definite size for the purpose of this section, since it
relates to sizing of children? Even though this would not normally be
considered to be a definite of the flexbox itself?

Thanks,
-Christian