Clarification of whether top-level fields in spec are optional or required

[keller-mark]

In reading the spec, it is helpful to know

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” are to be interpreted as described in RFC 2119.

However these keywords seem to only be used for the inner fields and seem to be missing for the top-level fields. It was not very clear to me whether (or which of) multiscales, axes, or coordinateTransformations are REQUIRED at the top-level for the root .zattrs of an OME-NGFF (Image) in v0.4.

The following text appears in the spec but does not use the keywords:

├── .zattrs               # Group level attributes are stored in the .zattrs file and include
│                         # "multiscales" and "omero" (see below).

Metadata about an image can be found under the "multiscales" key in the group-level metadata.

"multiscales" contains a list of dictionaries where each entry describes a multiscale image.

I guess the statement "can be found" means "can always be found" but using the keywords here would eliminate this guessing. It also seems that the answer differs based on whether the Zarr store conforms to the Image vs Plate schema.