to track polylines you have to set the variable plinetype = 1 otherwise 2d polylinee are drawn, to transform existing ones you can use converts or converts.
2d polylines were born with autocad, from the r13 version (1994) the lwpolilinee were introduced, i.e. optimized polylinee also called "read", that the property table displays you as polylinee.
if you edit one or the other (editpl) are however transformed into lw, while if you turn them into spline become 2d.
for the common mortal does not change anything, if not the fact that with the lw you can select "new" the vertices already outlined, while if you use programs in lisp it is easy that some do not work with the 2d.