As I previously stated, the circle of fifths is essentially a study in Dominant-Tonic relationships, with each note of the circle resolving to its nearest counter-clockwise neighbor. With this logic, we can assume that any note functioning as a Tonic is also capable of functioning as the Dominant to its nearest counter-clockwise neighbor, the Subdominant.
The terms Tonic-Dominant-Subdominant can be used to describe modulation between tonal centers, to identify the degrees of a scale, and to name chords in a progression. There’s really no place that these three words are not applicable in the entirety of western music, in everything from Bach to the Blues.
The Subdominant is surrounded by a bit of ambiguity, and maybe even a bit of controversy. By ascending clockwise around the circle of fifths from any pitch, six successive notes can be octave-reduced to form a major mode with the exclusion of only one missing note: the Subdominant. The Subdominant resides below the tonic and does not come from the same overtones/partials as the other pitches produced in a major scale. Because of this, the Subdominant has a tendency to exist through a change in key rather than as a chord that belongs to the Tonic. The I chord can be thought of as a chord that is shared with the subdominant key rather than the IV chord being a part of the tonic key.