From Creatures Wikia
Its syntax is:
BRN: SETT (command) TRACT_NUMBER (integer) LINE_NUMBER (integer) NEW_VALUE (float)
Tract numbers (and lobe numbers) are determined by a combination of order in the genome and age of activation, and start from 0. That is, if there are genes for brain tracts, call them Alpha, Beta, and Gamma, in that order. Alpha and Gamma activate at Embryo, and Beta activates at child. Alpha's id is 0, Gamma's id is 1, and Beta's id, when Beta comes to exist, will be 2.
SV Rule numbers are like this: Initialization SVRules are 0-15, Update SVRules are 16-31.