User`s guide

Model Execution
6-7
Note that the multitasking execution assumes that all tasks are multiples of
the base rate. Simulink enforces this when you have created a fixed-step
multitasking model.
The multitasking execution loop is very similar to that of singletasking, except
for the use of the task identifier (tid) argument to ModelOutputs and
ModelUpdate. The
ssIsSampleHit or ssIsSpecialSampleHit macros use the
tid to determine when blocks have a hit. For example, ModelOutputs (tid=5)
will execute only the blocks that have a sample time corresponding to task
identifier 5.