SID?

SAP SID- SAP System Identification

• It is a system identifier with three-digit code
• Should be in upper-case only
• SID uniquely identifies each system

Please refer with the below mentioned links

https://sapbasisblog.de/2019/03/25/reserved-sap-system-identifiers-sapsid/

https://service.sap.com/sap/support/notes/1979280