An STL file saves object data in a format used for stereolithography. STL files have a file-name extension of .stl. There is an ASCII STL format as well as a binary STL format. You can choose which to create when you export a 3ds Max scene.
STL files are generally used for purposes of rapid prototyping. A variety of methods use the STL format to construct prototypes. For example, many STL machines use a liquid polymer and harden it in small slices, creating a solid plastic model. Other STL machines use metal powder to create a model in steel. Still other machines use a special wax.
An STL object must define a complete and closed surface. You can check whether the geometry you want to export satisfies this criterion by using the STL Check modifier.
You can also import STL files. See Importing STL Files.