Deep and out of position, the small blind stops fighting for initiative and starts fighting for nut potential.
The 3-bet range condenses toward hands that welcome a giant pot; almost everything else that plays well flats. Bloated pots with dominated hands are how 500bb stacks vanish.
Small blind against a cutoff steal: mostly 3-bet or fold. Flatting builds multiway pots you play blind-first for the rest of the hand.
Assumes a ~4bb open; 3-bet to ~3x the open in position, ~4x from the blinds — larger as stacks deepen.