Data Logging
By recording information produced by the engine and transmission sensors, a data-logging programmer or box can reveal how the engine reacts to the tunes you install. This can be used to modify your tuning to match the needs of the engine.
Custom programming
While most boxes and programmers come with preset horsepower levels, some allow you to create custom tunes. This can be helpful in the quest for maximum power and should give you the flexibility to upgrade components like the turbo and injectors while maintaining tight control of the engine and transmission operations.
There's no telling what effect new '07-emissions equipment will have on the aftermarket. Will the engine still run if this diesel particulate filter is removed? Will programmers or boxes be able to sufficiently lie to the new computers so they won't freak out when emissions output is out of spec? | 
Some programmer/box setups will also control the electronics in your automatic transmission, but modules dedicated to operating the transmission can also be used. |
Chad Simper, our Diesel Power Challenge West alternate competitor brought along a second ECU for his Duramax. He swapped the computer just before the sled pull to take advantage of its pulling-specific programming. | |
Display
Displays range from those with small two-color screens to full-color touchscreen units. They can provide instant feedback about engine conditions and be used as an interface to change tuning settings on the fly while you are driving.
Even if the programmer/box you choose does not have a display, you need a way to watch your exhaust temperatures and other readings to avoid engine and transmission damage. At the very least, you'll need an EGT gauge to make sure temperatures don't go past 1,250 degrees F. Many companies offer display units that can work in conjunction with a programmer/box so fueling is dialed back when the EGT gets too high.
Quarter-mile and 0-60-mph timer
A fun feature of some units is dragstrip simulation. Using information from the speed sensor, a programmer/box can calculate how long it takes your truck to run from 0-60 mph and to 1,320 feet.
During our gas-versus-diesel test, we watched as the Granatelli Motor Sports crew loaded a program into a Ford 6.0L ECU with a Fuego tuner. They then stacked their Big G box on top of that. The result was a bone-stock Super Duty that made nearly 500 hp at the wheels.