How to make symbols with Mapserver?

Question

I want to convert some Mapinfo styles to Mapserver.
Here, I found some examples on how to make symbols. But I don't understand how it works.

For example a dashed line:

SYMBOL
 NAME "mapinfo-pen-73"
 TYPE vector
 FILLED true
 POINTS
  5 25
  25 30
  45 30
  35 25
END # Points
ANCHORPOINT 0.5 0
END # SYMBOL

And i get my dashed line.

What does the POINTS block mean? I can't find any explanation, how to write POINTS by self. Where i can find documentation?

UPDATE

After i'd readed @user30184 and @Andreas answers i'm steal dont understand POINTS block.

I'm looking on my example in map:

i see dashed line.
If POINTS it is a polygons shape it's look like:

This moment dont clear for me. Why shape form different with dashed like form?

Im missed with Y axe. But anyway shape form very strange.

Answer 1

How to construct symbols for MapServer is explained in the document that you have been reading. http://www.mapserver.org/mapfile/symbology/construction.html.

How to use POINTS is written rather unclearly

For symbols of TYPE vector and ellipse the shape of the symbol by setting X and Y values in a local two dimensional coordinate system with X values increasing to the right and Y values increasing downwards. The coordinates defining the symbol is listed in the POINTS parameter, which is explicitly ended using END. Negative values should not be used.

Example should clarify the usage. Let's make an unfilled triangle symbol with corners at coordinates (0 0), (2 0), and (2 1).

SYMBOL
  NAME "triangle"
  TYPE vector
  POINTS
    0 0
    2 0
    2 1
    0 0
  END # POINTS
END # SYMBOL