By "hex keypad", I'm assuming that you mean a keypad with buttons for 1, 2, 3, 4, etc. along with buttons "A" through "F"?
By "Applications" I assume you mean "What do you do with a hex keypad."
If this is what you mean, then the applications are as varied and broad as your imagination. Most of the times that I have used a hex keypad, it was just a quick source to input information because we didn't have any better keypad to work with. Also, the output from a hex keypad is easy to work with for beginning electronics hobbyists. It sounds to me like you are working with some kind of hobbyist electronics or programmable microcontroller.
Or perhaps there is some other definition of "hex keypad" this that I'm not aware of.