Thank you for your interest. Have you read what is written in the help file, under the MODEL command (http://t-t.dk/gekko/docs/user-manual/in ... ?model.htm
)? In general, the first letter after the '_' is only a label ('i' for identity, 's' for stochastic, etc.) that does not do anything while solving. But I believe the rest of the codes are explained under MODEL.
You do not have to use these codes, if in doubt just use a '_i' to avoid these complexities.
Also, I recommend trying out the guided tour (see section 5 in particular): http://t-t.dk/gekko/guided-tour
-Thomas Thomsen, editor/Gekko