Expanding the References field category in the field chooser will display master forms that are directly referenced from the master form you are editing.
Field chooser highlighting references category
By clicking one of the offered forms (image below) you can "reach through" to that master form. From there the fields and definitions available on that master form can be choosen. This "reach through" can daisy-chain through multiple forms if each has references to other forms.
Field chooser showing form references found on this master form
Field chooser showing fields available by "reaching" through a reference to equipment master form