When comparing the two, the difference is internal vs. external.
Fi: internal statics of fields
Ti: external statics of fields
External properties are things that can be observed or measured. Internal properties cannot be readily observed or measured. "Statics of fields" can be read "states of relationships" or "stationary connections".
This is why Fi can look arbitrary to someone who values fixed systems that are observable or measurable (Ti). Fi values are instead internal, unique to each individual. There's more to Fi than it's own value system though. It's also about "hidden" or "beneath the surface" relationships between other people/things. In fact, Te types value their duals for knowing them better than they know themselves (in terms of likes and dislikes, among other things).