Substitution strings are part of an internal APEX engine mechanism, which allows us to use pre-defined strings, and substitute them with their value, as part of the page rendering process.
The Application Builder supports three types of substitution strings: APEX engine built-in substitution strings, application substitution strings, and the application or page items.
Substitution strings must be named using CAPITAL LETTERS only. In any other notation, the APEX engine will treat them as regular strings. ...