Based on the experiments, it appears that the function f(x) is selective for two main concepts: celestial objects and colors. The values are higher for words related to these categories when compared to other words:

Celestial objects: sun, moon, star, comet. 
Colors: red, green, blue, yellow, black, white.

[DESCRIPTION]: The function f(x) is selective for words associated with celestial objects and colors, returning higher values for words belonging to these categories.