A solid modeller defines a mass having an interior that is completely enclosed and is filled matter.
Parts are always complete and have continuity when defined by a solid modeller.
A surface modeller builds a number of mathematical patches or surfaces that, when joined together, form the desired part. Because there is no defined association between these surfaces, it is possible to have gaps, discontinuances or even completely missing surfaces. To use a surface modeller for StereoLithography, the model must be 'fully surfaced', i.e. it must have a closed surface that completely defines an enclosed volume and the inside, outside and boundary of the part must be specified. Under-faceted STL files will affect the accuracy and may also affect the appearance of the SLA part. Over-faceted STL files will increase the time it takes to prepare the part, will not improve the quality of the final part, and may affect the delivery date.