Substance Painter real-time subsurface implementation is a screen-space subsurface scattering effect. The parameters to control it are explained in this page.
The current implementation is based on the "Approximate Reflectance Profiles for Efficient Subsurface Scattering" method published by PIXAR.

For examples of materials based on these parameters, see : Subsurface Material Type

Shader/MDL Parameters

Translucent: suited for generic materials such as Jade or Marble where light can penetrate deeply into an object.
Skin: suited for organic skin, where light is absorbed quickly and only scatter near the surface.

Available in the Shader Settings window.

Setting	Description
Enable	Activate or Deactivate the Subsurface Scattering effect on this shader/mdl instance. Can be used to disable the SSS effect on material that don't need it.
Scattering Type	Defines the behavior of the light absorption in the material : Translucent : suited for generic materials such as Jade or Marble where light can penetrate deeply into an object. Skin : suited for organic skin, where light is absorbed quickly and only scatter near the surface.
Scale	Controls the radius/depth of the light absorption in the material. Note : this parameter behavior change depending of the size of the mesh in the scene. Comparison between a scale of 0.0, 0.2 and 1.0 on a human sized head :
Color	The color of light when absorbed by the material. Comparison between three colors :

Display Settings Parameters

Available in the Display Settings window.

This parameter only affect the realtime version of the subsurface scattering effect.

Setting	Description
Sample Count	Controls the amount of samples that will be performed to generate the Subsurface blur in screen-space. More samples means less noise but will impact performances. Comparison between 8, 32 and 64 samples when looking close to a surface : The amount of noise can also be reduced by enabling Temporal Anti-Aliasing without increasing the amount of samples.