Stencil and depth buffers - more begginer's qustions

#1

From documentation , loosely quoted
" …stencil (buffer) is special case of depth (buffer) …"
" set stencil buffer pixel to 1 where you want decision to be made…"
So - if I want “flat” 2D stencil “action” do I have to specify “depth” ?
It looks as stencil with depth specify 3D “pixels”,

Example
if I want 2D “meteor trace” when meteor enters "atmosphere ( stencil buffer set to 1 ) area between x and y kilometers only " do I have to specify the area as 2D region?
GL_LINE at size x seems to specify sort of "donut / torroid " 3D region.

#2

SOLVED

Was missing
glutInitDisplayMode(
// GLUT_RGBA | GLUT_ALPHA | GLUT_DOUBLE | GLUT_DEPTH | GLUT_STENCIL); BEFORE the main window is created.

it is FINALLY working as expected (2D)- hence without using depth test,

It is still unclear how double buffering works in comparing color buffer and stencil buffer , if it really has any role at all.
The final “object” is rendered without glFlush nor glutSwapBuffers.