This kind of data object represents the simulation box information loaded from the input file. You can access this information by selecting the corresponding entry in the pipeline editor (see screenshot). Note that a corresponding visual element for the simulation cell exists in OVITO, which controls the visualization of the cell as a wireframe box (separation of data and visual representation).
Not all file formats contain information about the simulation cell that was originally used in the simulation. In such a case, an axis-aligned bounding box is automatically generated by OVITO to fully enclose all loaded particles.
The dimensionality setting determines whether the particle system is 2- or 3-dimensional. In "2D" mode, the z-coordinates of particles and the third simulation cell vector will be ignored in most computations performed by OVITO.
The periodic boundary condition (PBC) flags control whether OVITO should perform calculation within a periodic domain or not. If possible, OVITO tries to read or guess these flags from the input file, but you can manually override them if needed.
The numeric input fields for the cell vectors allow you to override the current cell geometry if needed. However, all cell vectors will be reset to their original values whenever OVITO reloads the input file. To permanently change the simulation cell geometry, you should make use of the Affine Transformation modifier instead.