The amount of amperage that your outlet can put out will determine how quickly your devices will be able to charge. The higher the amp, the bigger the device can be. Each USB port will draw a specific number of amps based on its max amp number. So, if an outlet has two ports and is listed as 4.0Amps, then each port will draw 2A each (unless specified otherwise). Sometimes the manufacturer will specify individual amperages per port.
There are six different AMP options to choose from: