Migration of legacy system is not a single step activity but a process that comprises of several phases of which Legacy System Understanding (LSU) is the first step. The intent of this work is to carry out a detailed study on the Legacy System Understanding in terms of Techniques and Tools used and to identify the potential gaps in them. The understanding of the legacy system has to be at the perspective of system level rather than the code level which has a narrow perspective, because the migration at code level may have a cascading impact to different aspects of the Legacy System. These findings have enabled us to formulate a process for building up an artefact repository and artefact dependency repository. These repositories along with the legacy system have aided us in understanding the legacy system in a comprehensive manner in terms of migrating artefacts in the context of migration of legacy systems. A formal mathematical model for representing the status of LSU and application of the same on a case study has been presented.