Rules for subdual are found on DMG 67 and DMG 72. Unconscious creatures are effectively held.

Subdual damage (the 75%) is healed at a rate of 1 hit point per round.¹⁾ Bedrest is not needed when 0 HP is reached via subdual.

When hit points are reduced to 0 or lower, the creature is unconscious. Unconsciousness lasts for 1 round, plus 1 round for every point of damage beyond 0 HP. For example, -4 HP would mean 5 rounds of unconsciousness.²⁾

In the paragraph about striking to subdue on DMG 67, we read:

The above, of course, does not apply to player characters.

Our interpretation of this sentence is that the DMG is saying that monsters are not likely to strike to subdue, NOT that PCs can't be struck with subdual damage. It might be argued that because of the definition of PC hit points on PHB 34, ALL damage before the final points is “subdual damage”. To illustrate the problem with this argument, consider a sword, cursed berserking. A cursed PC's fellows may ask how they can use their weapons to put the cursed PC down without killing them. We believe if PCs are capable of striking to subdue humanoid monsters, they should be able to strike to subdue PCs in such cases.