There are currently two stencil bits exposed to us.
https://github.com/Unity-Technologies/Graphics/blob/master/com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/HDStencilUsage.cs
Which bits are used for what
https://github.com/Unity-Technologies/Graphics/blob/master/com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/HDStencilUsage.cs.hlsl
Still a lot of work making it useful. This will most likely change with the Shader API update for both SRPs
There are big changes coming, to find out more read this post by the devs