The RBE of a single Bloon is the health of its outer layer plus the sum of the RBE of all its children, if any. If a Bloon spawns no children when popped, its RBE is simply its health.
In other words, it is the total amount of damage necessary to fully destroy a Bloon and all its children. The RBE of a round or a group of Bloons is the combined RBE of all Bloons present in that round or group. The Regrow and Camo properties do not impact its RBE, while the Fortified property increases it.
Mechanics
Bloons TD series
In the Bloons TD series, the number of lives lost upon leaking a Bloon is usually equal to the RBE of the Bloon when at full health. Boss Bloons are an exception, as the player always loses all of their lives should one leak - no matter how many lives were accumulated.
Bloons TD 4
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Bloons TD 5
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Bloons TD 6
In Bloons TD 6, there is no longer an in-game indication of how much RBE a particular round has. Several tower mechanics depend on RBE:
The Bloon Trap, XXXL Trap, and Obyn Greenfoot's Wall of Trees have a specific capacity of RBE it can capture at a time, becoming full and ready for collection once they reach or exceed capacity
XXXL traps can capture a MOAB-Class Bloon that would fill it above capacity, but Wall of Trees cannot
