Revival Protocol is an example of poorly worded homebrew feature

Dark Matter is not an official source book. It wasn't thoroughly playtested and can miss details.

Revival Protocol is a typical example:

Revival Protocol (3/Day). The drone casts the cantrip Spare the Dying on a target it can reach, then restores 1d4 of the target's hit points.

If Spare the Dying and healing were unrelated, the cantrip makes no sense. Healing itself stabilizes the target^*:

Stabilizing a Creature
The best way to save a creature with 0 hit points is to heal it.

Which means authors probably meant one of the following:

• You can use Revival Protocol only on targets with 0 HP
• You can use Revival Protocol only if Spare the Dying can be cast on the target
• Both are true

Unfortunately, the authors didn't specify this explicitly. As written, Revival Protocol is just a free healing 3 times per day, and Spare the Dying cantrip is irrelevant, which looks weird.

As a DM, you should make a ruling and stick to it. Based on the name, I'd say that Revival Protocol is meant to be improved Spare the Dying. So it should work only on 0 HP targets, and has no effect on undead or constructs.

^* Healing stabilizes the target only when it adds hit points. If the target can't regain hit points (see chill touch), it can't be stabilized via healing. In this case Spare the Dying actually makes sense on its own. Kudos for @Mołot for pointing this in comments.

Revival Protocol is an example of poorly worded homebrew feature

Dark Matter is not an official source book. It wasn't thoroughly playtested and can miss details.

Revival Protocol is a typical example:

Revival Protocol (3/Day). The drone casts the cantrip Spare the Dying on a target it can reach, then restores 1d4 of the target's hit points.

If Spare the Dying and healing were unrelated, the cantrip makes no sense. Healing itself prevents the target from dying^*:

Stabilizing a Creature
The best way to save a creature with 0 hit points is to heal it.

Which means authors probably meant one of the following:

• You can use Revival Protocol only on targets with 0 HP
• You can use Revival Protocol only if Spare the Dying can be cast on the target
• Both are true

Unfortunately, the authors didn't specify this explicitly. As written, Revival Protocol is just a free healing 3 times per day, and Spare the Dying cantrip is irrelevant, which looks weird.

As a DM, you should make a ruling and stick to it. Based on the name, I'd say that Revival Protocol is meant to be improved Spare the Dying. So it should work only on 0 HP targets, and has no effect on undead or constructs.

^* Healing stabilizes the target only when it adds hit points. If the target can't regain hit points (see chill touch), it can't be stabilized via healing. In this case Spare the Dying actually makes sense on its own. Kudos to @Mołot for pointing this in comments.

Revival Protocol is an example of poorly worded homebrew feature

Dark Matter is not an official source book. It wasn't thoroughly playtested and can miss details.

Revival Protocol is a typical example:

Revival Protocol (3/Day). The drone casts the cantrip Spare the Dying on a target it can reach, then restores 1d4 of the target's hit points.

If Spare the Dying and healing were unrelated, the cantrip makes no sense. Healing itself stabilizes the target:

Stabilizing a Creature
The best way to save a creature with 0 hit points is to heal it.

Which means authors probably meant one of the following:

• You can use Revival Protocol only on targets with 0 HP
• You can use Revival Protocol only if Spare the Dying can be cast on the target
• Both are true

Unfortunately, the authors didn't specify this explicitly. As written, Revival Protocol is just a free healing 3 times per day, and Spare the Dying cantrip is irrelevant, which looks weird.

As a DM, you should make a ruling and stick to it. Based on the name, I'd say that Revival Protocol is meant to be improved Spare the Dying. So it should work only on 0 HP targets, and has no effect on undead or constructs.

Revival Protocol is an example of poorly worded homebrew feature

Dark Matter is not an official source book. It wasn't thoroughly playtested and can miss details.

Revival Protocol is a typical example:

Revival Protocol (3/Day). The drone casts the cantrip Spare the Dying on a target it can reach, then restores 1d4 of the target's hit points.

If Spare the Dying and healing were unrelated, the cantrip makes no sense. Healing itself stabilizes the target^*:

Stabilizing a Creature
The best way to save a creature with 0 hit points is to heal it.

Which means authors probably meant one of the following:

• You can use Revival Protocol only on targets with 0 HP
• You can use Revival Protocol only if Spare the Dying can be cast on the target
• Both are true

Unfortunately, the authors didn't specify this explicitly. As written, Revival Protocol is just a free healing 3 times per day, and Spare the Dying cantrip is irrelevant, which looks weird.

As a DM, you should make a ruling and stick to it. Based on the name, I'd say that Revival Protocol is meant to be improved Spare the Dying. So it should work only on 0 HP targets, and has no effect on undead or constructs.

^* Healing stabilizes the target only when it adds hit points. If the target can't regain hit points (see chill touch), it can't be stabilized via healing. In this case Spare the Dying actually makes sense on its own. Kudos for @Mołot for pointing this in comments.