From Creatures Wikia
STIM WRIT is a CAOS command that has been around since C1. It sends a stimulus to a specific creature, usually when the object is activated, hit, or eaten.
In C1, the syntax for it was as follows:
stim writ from 10 255 0 0 35 250 57 50 58 100 44 20
The first number, 10, is to do with the importance of the stimulus. A higher number means the stimulus is more important.
255 0 is the neuron that will be adjusted and how much by. These numbers mean that no neuron will be stimulated.
the next 0 is a 'bit record of features'.
the last 8 numbers are the chemicals and amounts of the chemicals that the stimulus injects into your creature.
In C2, the syntax for using it was as follows:
stim writ from 0 255 0 0 37 150 71 150 73 150 0 0
which translates to stim the from, don't touch the brain, but give 150 units of hunger decrease, 150 units of fatty acid, 150 units of starch, and 0 units of no chemical.
an alternate syntax, most often used in C3/DS, could be
stim writ from 97 1
which in C3/DS translates to "stim the from, let the creature learn from this, and give 1x the chemicals determined by the 'I have played with a toy' stimulus".