Fi is not necessarily just about judgements regarding people but that is where Fi is most characteristic.
Fi types are most confident in evaluating reality in terms of connections between specific people - like laser beams connecting them.
Let us imagine that there are indeed laser beams connecting individuals. Everyone posses both a laser projector and a detector of other people's beams.
When a laser beam is established between two individuals, information between them is exchanged. If there is agreement on a set of criteria between the two individuals, the laser beam will turn, let us say, blue. And if there is a great lack of agreement on such criteria, the laser beam will turn red. Intermediate levels of agreement lead to intermediate colors (yellow, green, etc). The color of the laser beams connecting them to other people is the most important reality of all for Fi types.
Fi types are static. Perhaps the beam connecting them to a specific person will "flicker" a bit before settling on a specific color, but, once it does settle, Fi types prefer, and expect, that the color remains unchanged - and all things being equal, it will remain unchanged. Since Fi's own detector-criteria do not change easily, neither will the color of the beam connecting it to another person's, since Fi types tend to assume that others' color criteria are also fixed. It takes some major upheaval to make Fi reboot its laser-beam color for an specific person, and once it has changed, another, perhaps even greater, upheaval to make the color change again or change back.
Fi types prefer to surround themselves with those people with whom they share a blue laser beam, that's when they feel most comfortable. They prefer to isolate themselves from those with whom they share a red laser beam - and once it's red, it's not impossible for it to change, even to blue, but it's not something that the Fi types expect to happen very frequently. A constant change of color would leave them confused and mistrustful of the quality of their detectors.
The color can be established, of course, even with someone the Fi type never met personally, and up to a point, with people they don't really know, such as public figures - whereas in that case the color is more easily rebooted when new information is received.
The Fi types' own sense of identity, however, is not determined by how many people they can connect with in blue: rather, their sense of identity is determined by the solidity of their own detector-criteria that establish the color. Those criteria are the essence of the Fi type's willingness to establish a blue laser beam with someone, they are based on the Fi type's idea of which kind of people they would like to have a green beam to begin with. Should those criteria change and the Fi type suddenly have red beams which previously were blue, the Fi type will sense that reality itself, the own self of being, has been violated.
We can refine this image further by adding that deep romantic feelings are blue with silver stripes, for instance.
The difference between the ISFj and the INFj lies essentially on which kind of software feeds their laser beam detectors. The ISFj's is fed by an observation, and memory, of real events and real actions as perceived by the ISFj. The INFj's is fed rather by the INFj's perception of potential qualities and alternative possibilities regarding the specific person. Both are static, not easily changing; however, since the INFj's criteria are based on a potentials as perceived by the INFj, they can "jump" from one static state to another with less trauma than for the ISFj, for whom reality should be more fixed.