In the standard sketch 3 types of digital I/O can be declared in the digital I/O area of the board. 


Figure no.  2 Digital pins 

The yellow part is the available area (pin 22..53) for digital inputs[1], digital outputs and the ultrasonic sensors. It is not mandatory to declare all types of I/O. The order of the I/O type is not mandatory, but the ranges of the similair I/O pins should fit together. Remark: the ultrasonic I/O takes for each sensor 2 pin’s in the order; trigger-, echopin. For the declaration of the pins and there tagnames it is important to let the number of pins to be equal with the tagnames. Per I/O type you have to declare; the number of pins, the first pin to start the range and the tagnames.  

[1] Be aware that for the digital input pins the pullup resistor is activated.  A not connected pin is high (switch to ground)