A system identifier is a document processing construct introduced in the HyTime markup language as a supplement to SGML. It was subsequently incorporated into the HTML and XML markup languages.
This is similar to the ORACLE_SID (System Identifier) which is also used when users want to switch between instances.
The value "TTTTSSSSS" means that the model number and plant of manufacture are not required to generate a unique system identifier.
Developers have contacted The Next Web to share references to a new iPhone identifier and the next big operating system update for the smartphone and tablet devices: iOS 7.