What does coordinate[pos=0.3] do in a draw command?

Question

MWE
documentclass[12pt,a4paper]{article}
usepackage[T1]{fontenc}
usepackage{amsmath}
usepackage{amsfonts}
usepackage{amssymb}
usepackage{pgfplots}pgfplotsset{compat=1.18}

begin{document}
begin{tikzpicture}
begin{axis}
[
    % explicitly specify axis limits
    xmin=0,xmax=1,ymin=0,ymax=1,zmin=18,zmax=26,
    xlabel={x},
    ylabel={y},
    grid,
    % do not clip the draw commands and clip each
    % addplot to its domain
    clip mode=individual,
]
% line on the xy plane (that way goes behind the surface)
% we mark a point at 30% of the line
draw[thick]  (0.4,0,18) -- (0.4,1,18) coordinate[pos=0.3](a);
% line  touching the parabola
draw[thick]  (0.4,0,18) -- (0.4,0.3,25-5*0.4*0.4-0.3*0.3) coordinate[pos=0.3](b);% coordinate on the black parabola
% main surface, semi-transparent
addplot3[blue, no marks, surf, domain=0:1, samples=10, opacity=0.9] {25-5*x*x -y*y};
% graph restricted at at y=0.3 (parametric curve format)
addplot3[ultra thick, no marks, domain=0:1, samples=10, samples y=1] (x, 0.3, 25-5*x*x -0.3*0.3);
% draw spline and angle label
% in and out are the angle at which the curve leaves the final and initial point
draw[thick,->] (a) to[out=100, in=-10]  node[midway, above right]{$alpha$} (b);

end{axis}
end{tikzpicture}
end{document}

What does coordinate[pos=0.3] do in a draw command? like does it have to do with somekind of panning in the graph?
Another question:
Tell me a wonderful way to put pgfplot examples in the public domain. I am thinking a GitHub repository such as APEXCalculus. Do any of you have a good idea?

Rmano · Answer

As a personal advice, you should really go through the tutorials in the Tikz manual, especially the first one; otherwise you will spend much more time with this kind of doubts. pgfplots is based on TikZ, and as you need basic arithmetic to understand algebra, so you should learn TikZ to use pgfplots or other libraries based on it.
Anyway
draw (0,0) -- (1,1);

draws a line from (0,0) to (1,1) in the current coordinate system.
draw (0,0) -- (1,1) coordinate(A);

gives a name to the coordinate (1,1) that you can reuse (so that for example, you can change coordinates in a drawing in just one place).
draw (0,0) -- (1,1) coordinate[pos=0.3145](B);

gives a name to the coordinate in the segment from (0,0) to (1,1) at the position 0.3145 (where 0 corresponds to (0,0) and 1 to (1,1)).
Example:
documentclass[border=10pt]{standalone}
usepackage{tikz}
begin{document}
begin{tikzpicture}[]
    draw (0,0) -- (1,1) coordinate(A);
    draw (0,-2) -- (1,-1) coordinate[pos=0.3](B);
    draw [red] (A) -- (B);
end{tikzpicture}
end{document}

What does coordinate[pos=0.3] do in a draw command?

MWE

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