this is some information about the meanig of the
path_attuned, path_repelled, path_denied fields.

up to date information from the historic documentation (updated a bit):
================================================================================
   3. SPELL PATHS DOCUMENTATION

   Long ago a number of archmages discovered patterns in the web that spells
   weave in the aether. They found that some spells had structural similarities
   to others and some of the mages took to studying particular groups of spells.
   These mages found that by molding their thought patterns to match the patterns
   of the spells they could better utilise all the spells of the group. Because
   of their disciplined approach, the mages were described as following spell
   Paths. As they attuned themselves to particular spell Paths they found that
   they would become repelled from others, and in some cases found they were
   denied any access to some paths. The legacy of these mages remains in some
   of the magical items to be found around the world.

   Technical details:
   In the same way that players and objects can be protected, immune, and
   vulnerable to the different types of attacks, they can now be attuned,
   repelled, or denied access to the different spell Paths. An object that
   is attuned to a Path cast spells from that Path at 80% of the spell point
   cost and receives duration/damage bonuses as if the caster were five levels
   higher. An object that is repelled from a Path casts spells from that Path
   at 125% of the spell point cost and receives duration/damage bonuses as if·
   the caster were five levels lower (minimum of first level). An object that
   is denied access to a Path cannot cast any spells from it. The casting·
   time is also modified by 80% and 125% respectively. These values are
   defined in PATH_SP_MULT (from spells.h), PATH_TIME_MULT (from spells.h), and
   path_level_mod (from spells.c, now called min_casting_level)
   ·
   The Paths themselves are the following:
     "Nothing",
     "Protection",
     "Fire",
     "Frost",
     "Electricity",
     "Missiles",
     "Self",
     "Summoning",
     "Abjuration",
     "Restoration",
     "Detonation",
     "Mind",
     "Creation",
     "Teleportation",
     "Information",
     "Transmutation",
     "Transferrence".

   See spells.h for the number values corresponding to the Path.

   Some more will be added in the near future. Some spells do not currently·
   belong to a Path, this is probably appropriate for some spells.
   Paths are inherited just like protection/immunity/vulnerability, ie if a
   ring contains "path_attuned 1", the wearer becomes attuned to the Path of
   Protection.

   Paths are quite powerful and shouldn't be given away cheaply. Ideally, most
   objects with path_attuned attributes should have path_repelled and path_denied
   attributes as well, to balance out (eg attuned to Fire, repelled from·
   Protection, and denied from Restoration)
================================================================================

This is the information i got from the source:

generally the server code does few things with the constants
defined by include/spells.h PATH_*. The main
behavoir is decided by the items the player is wearing
and other things that alters his pathes.

generally:
  - the spell can't be spoken or written in a rune
    if player->path_denied & spell->path_attuned

  - if player->path_repelled & spell->path_attuned then the
    spell's level is raised by 2 in the calculation of the minimal skill level
  - if player->path_attuned & spell->path_attuned then the
    spell's level is lowered by 2 in the calculation of the minimal skill level

  - if player->path_repelled & spell->path_attuned then the
    players skill level is decreased by 2
  - if player->path_attuned & spell->path_attuned then the
    players skill level is increased by 2

  (this split of the calculation is done because the adjustment of the minimal
  skill level is also used to calculate the damage/duration/range adjustment
  of the spell)

- dragons have a skin which has path_attuned
- if player->path_denied & PATH_LIGHT: can't make objects glow

for SP_MOVING_BALL: if the spell->path_repelled is not 0 and
the player isn't attuned to all paths in the bitmask of spell->path_repelled
he can't cast this.