I am interested in the frequency function. Why is there a % symbol next to the Hz setting on the multi-meter selector dial?

The meter also has a "Duty Cycle" function and this is incorporated with the Frequency Function button. The duty cycle is particularly useful when dealing with pulsating circuits. The pulse on/off ratio is displayed as a percent, hence the % sign on the button.

its the duty cycle %. Duty cycle is the proportion of time during which a component, device, or system is operated. The duty cycle can be expressed as a ratio or as a percentage. Suppose a disk drive operates for 1 second, then is shut off for 99 seconds, then is run for 1 second again, and so on. The drive runs for one out of 100 seconds, or 1/100 of the time, and its duty cycle is therefore 1/100, or 1 percent.