Three equal masses, Newtonian gravity, G = 1. The general three-body problem has no closed-form solution, but a handful of exact periodic orbits are known, and the first four presets are some of them: the figure eight of Moore, Chenciner and Montgomery, and three of the Šuvakov–Dmitrašinović family. Preset 5 is Lagrange's rotating equilateral triangle, which is unstable for equal masses — watch it come apart. Preset 6 throws the bodies in at random.
The integrator is RK4 with a step that shrinks during close approaches. ΔE/E₀ is how far the total energy has drifted from its starting value: the honest measure of how much of what you are watching is physics and how much is arithmetic.