Grouping statements set a state: they apply to every element after them, until the next statement of the same kind. They do not change the vertex numbering.
g – Group
g name1 [name2 …] names the groups the following elements belong to. An
element can be in several groups at once. A file with no group statement puts everything in
the group called default.
s – Smoothing group
s number or s off. Elements with the same non-zero number
are smoothed together where they share an edge. 0 and off
turn smoothing off. For free-form surfaces any number above 0 means on. A face with
normals of its own ignores smoothing groups.
mg – Merging group
mg number res or mg off. Free-form surfaces in the same
merging group are checked for adjacency within the resolution res and
joined where they meet. res must be greater than 0 and is required when a
group is set. 0 and off turn merging off.
o – Object name
o object_name names the object the following elements belong to. The
specification says this name is informational only and is not processed.
Many programs write one o for each separate part of a model. The
explorer uses o, then g, to arrange a file’s structure tree.
o Table
g top
s 1
usemtl Oak
f 1 2 3 4
g legs
s off
usemtl Steel
f 5 6 7 8