To exchange the coupling fields going through OASIS3 main process (i.e. with status EXPORTED, AUXILARY, or EXPOUT, see section 5), in a given order at each coupling timestep, a sequence index SEQ must be defined for each of them. This is not required for I/O fields or for coupling fields exchanged directly between the component models, i.e. with status IGNOUT, INPUT or OUTPUT. SEQ gives the position of the coupling field in the sequence.
A coupling algorithm, showing the SEQ concept, is illustrated on figure 4.6. All coupling field produced by the source model at the coupling timestep can be ``consumed'' by the target model at the same timestep without causing any deadlock situation; therefore, LAG = 0 for all coupling fields. However, at each coupling timestep, a particular order of exchange must be respected; must be received by model A before it can send , which in turn must be received by model B before it can send . Therefore, SEQ = 1, 2, 3 must be defined respectively for , and . As all fields can be consumed at the time they are produced (LAG=0 for all fields), there no reading/writing from/to coupling restart files.