Molecules are often represented as line formats such as InChI or SMILES strings. These formats provide information on atom connectivity, but no distances, angles, or Cartesian coordinates. When a molecule is added to the database via such a format, 3D coordinates will typically be generated automatically and asynchronously (using Open Babel by default). However, it is possible to skip 3D coordinate generation, and keep only the line format and a 2D preview. This can be particularly useful for running large sets of calculations that do not require 3D coordinates (such as machine learning algorithms), since generating 3D coordinates for many large molecules can be computationally expensive.
For visualization, if a molecule does not have 3D coordinates, SVG images generated with Open Babel are used where 3D rendering would typically be used. See the image below for an example.