SFDebris' theory was of different models to act as a general 'theme' for the Collective.

'First Contact' was strategic, take down an unexpectedly resilient and dangerous threat to the Collective.

'Dark Frontier' was aggressive, following on from the events of Scorpion. Bold expansion and unsubtle strikes.

Then when she royally screwed up in Unimatrix Zero, that one was retired to return to the more reserved and strategic First Contact model.