externalConstant
A reference to an externally supplied constant value, prefixed with %. The name may be a keyword identifier, a plain identifier, or a string literal. External constants are resolved by the evaluation environment at runtime.
Examples
External constant by identifier
%measurementPeriod
External constant by string name
%'measurement-period'
Used in an expression
define "Period": %measurementPeriod
EBNF
externalConstant : '%' (keywordIdentifier | identifier | STRING) ;