Tablets are built by placing a tablet case into an assembler, configuring as desired and assembling it. Tablets act as portable computers that cannot directly interact with the world - for example, basic redstone cards do not work in them. A number of upgrades do, such as the sign i/o upgrade or the piston upgrade.
The tier 2 tablet also allows installing a single container upgrade. The slot provided by the container can be accessed by opening the tablet's alternative GUI, by shift-right clicking with the tablet in hand. This will also forcibly shut down the tablet.
Unlike computers, tablets do not persist across the player holding it leaving and re-entering the game. They also do not persist across dimensions (such as a player going to the Nether and back).
Tablets can be put into a charger to refill their energy, and to access the first hard drive in the tablet from a computer connected to the charger - in this setup, the charger will behave similar to a disk drive, with the tablet being the floppy disk. This can be very useful in case you forgot to install an OS on the hard drive built into the tablet, or after bricking a tablet's OS.