The twelve keyboard keys F1 through F12 as well as their Ctrl+, Shift+, Alt+ variants are called hot-keys. Any active skill or item may be assigned to a hot-key to facilitate access to that feature.
With 48 hot-keys available for your character the possibilities for a custom interface are very large indeed.
The twelve keys 1,2,3,4,5,6,7,8,9,0,-,= (but 'not any of the shifted variants) are available for assigning Active skills for your summoned creature.