Currently users must use M1, M2, M3, or M4 as memories for custom expressions and f, g, or h for custom functions. Although this works, it is less than ideal. Users can't name their expressions or functions something more meaningful and they are restricted to four expressions and three functions.
I am working on allowing users of the paid version to create custom keys for their custom expressions and functions. You will be able to use letters in the English and Greek alphabet to name your custom expressions and functions. You will also be able to create as many as you wish.
For example, someone my use the golden ratio a lot. It is usually represented by the Greek letter φ (phi) and is approximately equal to 1.6180339887. With this new feature, you could create a custom key named φ and set that equal to 1.6180339887. The key could be accessed by the "more" key or you could place the φ key on a custom keyboard to make it easier to access.