contextValue 
  Context value of the resource state. This can be used to contribute resource specific actions. For example, if a resource is given a context value as diffable. When contributing actions to scm/resourceState/context using menus extension point, you can specify context value for key scmResourceState in when expressions, like scmResourceState == diffable.
"contributes": {
  "menus": {
    "scm/resourceState/context": [
      {
        "command": "extension.diff",
        "when": "scmResourceState == diffable"
      }
    ]
  }
}Content copied to clipboard
This will show action extension.diff only for resources with contextValue is diffable.