UnityCACAO-HDRP17/com.unity.render-pipelines.high-definition/Documentation~/snippets/shader-graph-blocks/ambient-occlusion.md

Ambient Occlusion The material's ambient occlusion. This approximates occlusion for a fragment on a GameObject’s surface that has been cast by details present in the Material but not the mesh geometry. A value of 0 means the fragment is completely occluded and appears black. A value of 1 means the fragment is not occluded at all, and the ambient color does not change. None 1.0