I submitted a SCATS application this year and you need to provide supporting evidence for whatever you’re claiming disadvantage for. When you’re on the SCATS application portal, they will tell you what type of supporting documentation is needed. For example, for financial disadvantage they will ask you to provide either Centrelink statements or bank statements- the personal statement is for you to describe the disadvantage and how it impacted your studies.
I’m not sure I understand what you mean by “written letters by you”, the personal statement should be the only part written by you- the supporting documentation must all be official documents (e.g. bank statement, psychologist report). Written statements, for example a psychologist report, should be signed.