A show package is just the agencies showing off their best-of-the-best when it comes to old classics and new faces. As for how they pick them- I suppose it's just sort of choosing the ones that a) have the most potential and b) have had the most success so far.
For example:
IMG will naturally have
Daria,
Liya, and
Gemma (who will all certainly book every major show they can possibly do) and then some girls that have had a lot of hype and a generally positive response so far (
Sonny- Asian models are doing very well right now,
Nadeja- looks a bit like Eugenia V., among others). Not to mention, girls that did well with other agencies (
Caroline Winberg,
Diana Dondoe, and
Maria Dvirnik)
And now I have a question: Did just about every successful model on the planet leave their agencies and go to IMG?! Seriously!