Hi Jcal,
The terminal blocks (SCB-68 or SCB-100) are both basically the same peice of hardware. They let you wire in signals to your daq board's cable. If you have a board with a 68 pin cable, you will need the SCB-68. If you have a board with a 100 pin cable, then you will need a SCB-100. The SCB breakout boxes are simply screw terminals that connect to your daq board's IO lines.
Configuration of channels is dependent on what your board can do. Whether or not you are capable of having 30 digital inputs and 18 digital output depends on which board you are using. The SCB-100 will be for boards with 100 pin connectors.
-Sal