The documents refer to different values for $expand: property names or paths ending in a navigation property. This needs to be aligned:

• Protocol spec: 11.2.5.2 System Query Option $expand: “The value of the $expand query option is a comma-separated list of navigation property names, stream property names, or $value indicating the stream content of a media-entity.”
• URL spec: 5.1.3 System Query Option $expand: “An expand item is either a path or … A path can end with … a navigation property to expand the related entity or entities”