Usually in speaker design it means that the designer stopped measuring and made tweaks based on subjective listening impression. Changing the "voice" of a speaker if you will.
This can be very small such as trying out a bypass capacitor, or bigger, like changing the tweeter to woofer balance until the designer likes what he / she hears.
The term makes most sense with loudspeakers, but can be applied to electronics.
Personally, once I found my favorite reference curve, that is my crossover design, with the exception of minute changes made by bypass caps.
Best,
Erik