What does this equation mean?

One thing holds regardless of which cell the industry lands in: the independence assumption behind $\mathrm{MTTF}_{\mathrm{fleet}}$ $\approx$ $\mathrm{MTTF}_{\mathrm{device}}$/N keeps breaking as fleets grow, whether the response is better synchronous engineering or a move away from synchrony altogether. Correlated failure — a bad manufacturing lot, a shared dependency, a batch of units commissioned and aging together — is documented independently by Meta’s silent-data-corruption study [ 12 ] and implicit in the taxonomy of failure types in Meta’s reliability study [ 3 ] . That pressure exists in every scenario above, which is why fault-domain design — structuring which units share which dependencies, so correlated failure stays contained rather than fleet-wide — sits underneath all five trajectories rather than being a sixth one.