What spatial reference for ERA5 data?

I'm relatively new to GIS and referencing, so I may misunderstand something.

I find the doc a bit confusing about the spatial reference to use.

Here it's said that "ERA5 data is referenced in the horizontal with respect to the WGS84 ellipse".

Paragraph below mentions a spherical model (so, not WGS84) and 2 different radiuses depending on GRIB version

And this page says we should use a 6367.47km sphere (and that this referencing has no EPSG).

I'm confused. What referencing should be used ? 

Also: does it depend on the download format (GRIB vs netCDF) ?


please use the WGS84 ellipse to interpret the latitude-longitude values.