If you save your STL in ASCII format, as opposed to binary, you can actually confirm this yourself. The format of STL files is very, very simple. It's just a list of vertices organized into structures like loops and facets, and normal vectors describing the orientation of these structures.
A simple 15mm, 1.5mm, 1.5", 1.5m (you get the idea

) cube would be composed of twelve triangular facets made up of 3-vertice edge-loops, the 6 surfaces of the cube being described by two of these facets per surface. Each facet also has a normal vector, that describes which side of the facet is the "outside".
Like so:
Code: Select all
solid ASCII
facet normal 0.0 1.0 0.0
outer loop
vertex 1.5 1.5 1.5
vertex 1.5 1.5 0.0
vertex 0.0 1.5 1.5
endloop
endfacet
facet normal 0.0 1.0 0.0
outer loop
vertex 0.0 1.5 1.5
vertex 1.5 1.5 0.0
vertex 0.0 1.5 0.0
endloop
endfacet
facet normal 0.0 -1.0 0.0
outer loop
vertex 0.0 0.0 1.5
vertex 0.0 0.0 0.0
vertex 1.5 0.0 1.5
endloop
endfacet
facet normal 0.0 -1.0 -0.0
outer loop
vertex 1.5 0.0 1.5
vertex 0.0 0.0 0.0
vertex 1.5 0.0 0.0
endloop
endfacet
facet normal -1.0 0.0 0.0
outer loop
vertex 0.0 1.5 0.0
vertex 0.0 0.0 0.0
vertex 0.0 1.5 1.5
endloop
endfacet
facet normal -1.0 -0.0 0.0
outer loop
vertex 0.0 1.5 1.5
vertex 0.0 0.0 0.0
vertex 0.0 0.0 1.5
endloop
endfacet
facet normal -0.0 -0.0 -1.0
outer loop
vertex 1.5 1.5 0.0
vertex 1.5 0.0 0.0
vertex 0.0 1.5 0.0
endloop
endfacet
facet normal 0.0 0.0 -1.0
outer loop
vertex 0.0 1.5 0.0
vertex 1.5 0.0 0.0
vertex 0.0 0.0 0.0
endloop
endfacet
facet normal 1.0 0.0 0.0
outer loop
vertex 1.5 1.5 1.5
vertex 1.5 0.0 1.5
vertex 1.5 1.5 0.0
endloop
endfacet
facet normal 1.0 0.0 0.0
outer loop
vertex 1.5 1.5 0.0
vertex 1.5 0.0 1.5
vertex 1.5 0.0 0.0
endloop
endfacet
facet normal -0.0 0.0 1.0
outer loop
vertex 0.0 1.5 1.5
vertex 0.0 0.0 1.5
vertex 1.5 1.5 1.5
endloop
endfacet
facet normal 0.0 0.0 1.0
outer loop
vertex 1.5 1.5 1.5
vertex 0.0 0.0 1.5
vertex 1.5 0.0 1.5
endloop
endfacet
endsolid