Sometime you might find you need a few more I/O than the 16 that comes with the T1-16S. There is an easy solution with cards that plug into the side of the unit.
Some cards use high density connectors so be sure you get the associated cables.
There are also adapter cards that allow the T1-16S to use T1-40S expansion cards.
Finally, you can even get a mini rack that allows you to use up to 4 T2 cards.