Extract from Fundamental Potential:

The Four Buddha Bodies. Through the continued development of the potential, finally, the two surpassing aspects of perfection will be attained. The two surpassing aspects of perfection are the surpassing perfect benefit for oneself, which is the attainment of the Truth Bodies of a Buddha, and the surpassing perfect benefit for others, which is the attainment of the two Form Bodies of a Buddha. The two Form Bodies of a Buddha are the complete enjoyment body and the emanation body. The two truth bodies are said to be subtle bodies because they cannot be seen by sentient beings. They are thus primarily of benefit for oneself and are referred to as Buddha's‘ bodies of benefit for oneself’. The two form bodies are called gross bodies (when compared to the truth bodies) because they can be seen by sentient beings as they display their actions of teaching the Dharma and so on. They are thus primarily of benefit to others and are referred to as Buddha's‘ bodies of benefit to others’. The division of the four bodies of a Buddha into gross and subtle is made on the basis of whether they can be seen or not by sentient beings. The visual consciousness of an ordinary sentient being is able to see Buddhas’ emanation bodies but not complete enjoyment bodies. The visual consciousness of a superior Bodhisattva sentient being is able to see complete enjoyment bodies. However, neither of these is able to perceive a Buddha’s wisdom truth body directly. Only a Buddha can perceive this directly. It is true, however, that a superior Bodhisattva is able to perceive both aspects of the Buddha’s natural body, the intrinsically pure nature body and the adventitiously pure nature body. However, because he does so only by way of mental consciousness and not visual consciousness, the division of the Buddha’s bodies into gross and subtle according to whether they are ‘seen’ by sentient beings or not remains accurate.

With thanks to the late Acharya Geshe Loden.