Cages can be removed and changed.

Short and medium cages are used with double and compact double chainrings, long cages are used with triple chainrings though a medium can be used.

You get quicker shifts with short cages than long but the difference is minimal, you need a laser stop light to be able to tell the difference!

The cage takes up the slack in the chain when you shift from the large 52 tooth chainring to the 30 tooth chainring. Siince there is less of a size difference between a 50 and a 34 or a 50 and a 36 than a 52 30, the cage does not need to be as long to take up the slack.