3  Mental model

The logical cube always has five axes, including singleton axes. A surface field uses one depth position (often NA) rather than dropping depth. Variables form a logical fifth axis even when a NetCDF stores them as separate physical variables.

ocean_cube
├── coordinates: longitude, latitude, depth, time, variable
├── metadata: units, extents, provenance, mask, QA
└── backend: memory | netcdf

A coordinate is a scientific value; an index is its integer position. Logical order is canonical; physical NetCDF order may differ and is translated by the backend. Missing values remain NA; units and provenance travel with derived cubes.