XPBD
Extended Position Based Dynamics — the addition that makes a spring's stiffness independent of how many iterations and timesteps you happen to run.
Plain PBD has an embarrassing bug: a rope gets stiffer the more iterations you run and the shorter the timestep. Stiffness is a side effect of solver settings, not a physical parameter you can dial in.
XPBD (Macklin, Müller and Chentanez, 2016) introduces compliance — inverse stiffness — directly into the constraint along with an accumulated Lagrange multiplier. The result is that a material of a given stiffness behaves the same regardless of iteration count or timestep. That made position-based physics credible enough for production and underpins Nvidia's cloth and soft-body solvers, among others.