Rooms with no naturally placed enemies. This includes "trap rooms" with Poky-type enemies, though deactivated pokies will spawn in their original location.
Respawned Mini-Bosses will always have the same drop.
When used in a room with a Champion enemy, the enemy will not respawn as a Champion.
Enemies respawn exactly where they first spawned, even if that's where Isaac is standing, in which case Isaac will take damage immediately upon use of the D7. Right next to doors is usually a safe place to use it.
If used in the Mirrored World of Downpour or Dross II, the rooms will not only have their enemies respawn, but all the normal non-mirrored contents such as pickups are restored. Using it in the Shop causes it to be stocked normally, and additional uses will restock the Shop's contents, unlike normal Shops.Using it in the mirrored treasure room that does not have the knife piece in Downpour XL / Dross XL will spawn an item.
In Mines or Ashpit, the D7 cannot regenerate minecarts. Any monsters that were riding a minecart on room entry will respawn on foot.
As Tainted Keeper, enemies respawned by the D7 will drop the same type of coins as they had before using the D7. This can be used to quickly generate money from enemies that dropped high-value coins or greatly increase Luck if an enemy dropped a Lucky Penny.
Enemies respawned by the D7 don't drop loot when killed if they were champions and don't grant score in daily runs.
Synergies
4.5 Volt: Depending on the enemies in the room, some rooms can grant enough charge to be rerolled indefinitely.
9 Volt/AAA Battery: Quadruple-sized or L-shaped rooms can be rerolled indefinitely. With both of them, even normal sized rooms can be rerolled indefinitely.
Book of Virtues: Spawns a grey six-sided die wisp on the middle ring. The wisps start gray; if the wisps get hit while they’re gray, they invoke the effect of the D7 in the current room and turn blue. Damage that turns the wisps from gray to blue is negated.
Interactions
Car Battery: Doubles the amount of enemies summoned, but does not give double rewards.
Prior to Repentance, this item's description was "Reroll Rewards!"
Bugs
Bug!
If used in a non-Boss Room containing Chubs, C.H.A.D.s, or Carrion Queens, each segment of the boss(es) will be revived as an entire clone, tripling their number.
Bug!
If used in a "trap room" with Grudges after all Pressure Plates were pressed, all Grudges will respawn and will remain active if the room is left and re-entered.
Poky-type enemies, though deactivated pokies will spawn in their original location.
Challenge Rooms and 
Boss Challenge Rooms.
Shopkeepers, obstacles or Angel statues do not count as naturally placed.
Boss Rooms.
Mega Satan fight.
Mother fight.
The Beast fight.
Teleports Isaac away and restarts another room in the level if used in 
Angel Rooms or 
Devil Rooms.
Secret Rooms and 
Shops that had a 
Greed or 
Super Greed fight into regular Secret Rooms and Shops.
Krampus can be fought again in the Devil Room, like other Mini-bosses.
Downpour or 
Dross II, the rooms will not only have their enemies respawn, but all the normal non-mirrored contents such as pickups are restored. Using it in the Shop causes it to be stocked normally, and additional uses will restock the Shop's contents, unlike normal Shops.Using it in the mirrored treasure room that does not have the knife piece in 
Mines or 
Ashpit, the D7 cannot regenerate minecarts. Any monsters that were riding a minecart on room entry will respawn on foot. 
Tainted Keeper, enemies respawned by the D7 will drop the same type of coins as they had before using the D7. This can be used to quickly generate money from enemies that dropped high-value coins or greatly increase Luck if an enemy dropped a Lucky Penny.
4.5 Volt: Depending on the enemies in the room, some rooms can grant enough charge to be rerolled indefinitely.
9 Volt/
AAA Battery: Quadruple-sized or L-shaped rooms can be rerolled indefinitely. With both of them, even normal sized rooms can be rerolled indefinitely.
Book of Virtues: Spawns a grey six-sided die wisp on the middle ring. The wisps start gray; if the wisps get hit while they’re gray, they invoke the effect of the D7 in the current room and turn blue. Damage that turns the wisps from gray to blue is negated.
Car Battery: Doubles the amount of enemies summoned, but does not give double rewards.
D10 and 
Dice Shard.
Bug!
Chubs, 
C.H.A.D.s, or 
Carrion Queens, each segment of the boss(es) will be revived as an entire clone, tripling their number.
Grudges after all 
Pressure Plates were pressed, all Grudges will respawn and will remain active if the room is left and re-entered.
Dice 
The Binding of Isaac: Rebirth
