Martin (1992: 5):
As noted above, system is related to process through the concept of realisation — realisation formalises the instantiation of the system in process.
 System is related to structure through the concept of realisation — not to the process of instantiation.
 Realisation doesn't "formalise" the instantiation of the system in process. Realisation is an intensive identifying relation (token-value) between levels of symbolic abstraction, as between strata and between axes; instantiation is an intensive attributive relation (token-type) between actual instances of the system and the overall system of potential (Halliday & Matthiessen 1999: 14-5, 145). See also here.