Hi Nick. While the UT4 may advertise 12 functions and the Zephyr advertises 8 functions, it is highly unlikely you will ever get anywhere near using either 8 or 12 functions. And, your use of the functions will depend on the capabilities of each decoder you install in your loco.
I'm not familiar with the functions on the Zephyr but I presume they are the same as with a DT300/ 400 or a UT4.
One of the functions included in most locomotive decoders is the lights. This is usually controlled by function F0. Your throttles definitely have an F0 or similar "turn the lights on or off" function. Another function with lights is to dim them (sometimes referred to on the railways as "Rule 17"). Your throttle will definitely have this function - probably F1. If you want to add gyrolights (assuming your decoder can operate them), this may be another function.
If you install a sound decoder, you will need two other functions - one to turn on the bell (usually referred to as a "latching" function), and another to blow the horn or whistle. These two functions are usually shown on the throttle as a bell and a horn.
So, we're now up to 4 functions and I'm running out of ideas for the other 4 or 8 functions. Also, your throttle may have the capability to reprogram (or reassign) functions. And, if you press a combination of two function keys, you can usually get another bunch of functions.
All of which to say that it is highly unlikely that you will come anywheres near close to using all of the functions listed on your Zephyr or your UT4.