Starting at 1st level, rangers gain one field and one study in that field. This reflects the knowledge that the ranger has been able to gather while studying their calling. This knowledge is then expanded as the ranger increases in level, for it is presumed that the ranger is naturally inquisitive, exchanging techniques with others, tending to their weapons, physically improving their bodies at every opportunity, growing more adept and able with respect to principles of war, training, managing animals, scouting and so on.
This is true even if the player never expresses their intention to gather knowledge, as not every moment of 'downtime' is actively accounted for during play.
Mastery of Arms