What do double square brackets around "position" mean in the Metal language?

The Metal Shading Language is documented at https://developer.apple.com/metal/metal-shading-language-specification.pdf

In particular look at "Table 9" on page 68 of that document. There [[position]] is identified as an attribute qualifier for the return type of a Vertex Function. I assume that means that when your vertex shader returns the caller will use the values in that part of the struct to determine the positions of the vertices the shader would like to modify.