The Dink Network

Dunno about the key scripts in freedink but sp_custom is just a way of storing some values for a sprite (it doesn't actually do anything as such)
Example from the DinkC Reference:

// Place '1' in the 'Pie' key
sp_custom("Pie", &current_sprite, 1);

// Place '0' in the 'Cake' key
sp_custom("Cake", &current_sprite, 0);

// Get the values
int &pie = sp_custom("Pie", &current_sprite, -1);
int &cake = sp_custom("Pie", &current_sprite, -1);

// Dink will say "1 - 0"

say("&pie - &cake", 1);