Since Level-1 S-functions have long been deprecated, and only exist for the sake of backward compatibility, there is not a lot of published documentation on them (if you are writing a new S-function, please consider writing it as a Level-2 S-function). However, if you are maintaining an existing level-1 S-function and want to understand what the return value (sys) means for each flag, type this command in MATLAB:
>> help sfuntmpl
There is a bunch of information. In particular, the part that you are asking about is as follows:
What is returned by SFUNC at a given point in time, T, depends on the
value of the FLAG, the current state vector, X, and the current
input vector, U.
FLAG RESULT DESCRIPTION
----- ------ --------------------------------------------
0 [SIZES,X0,STR,TS] Initialization, return system sizes in SYS,
initial state in X0, state ordering strings
in STR, and sample times in TS.
1 DX Return continuous state derivatives in SYS.
2 DS Update discrete states SYS = X(n+1)
3 Y Return outputs in SYS.
4 TNEXT Return next time hit for variable step sample
time in SYS.
5 Reserved for future (root finding).
9 [] Termination, perform any cleanup SYS=[].
