Additive manufacturing always starts from an idea captured as geometry before any physical building happens. First the part is designed as a solid model in CAD software. That solid model then has to be converted into a format the printer can understand, so it is tessellated into a mesh of small triangles and saved as an STL file. Only once the STL file exists can the machine actually build the part, laying down material layer by layer. After the part comes off the machine it still needs cleaning up, support removal, or curing, which is the post processing stage. So the order a physical part passes through is CAD design, then STL conversion, then build, and finally post processing.