ModeShape 3 configuration files are JSON files, and ModeShape defines a JSON Schema for them. The configuration files need to allow system properties to be specified in the same format as in 2.x (see MODE-1098), namely using "${propertyName}" within a value. Multiple properties can be named within the variable, as can a default. Here's an example of a variable with two property names and a default:

${propertyName1,propertyName2:defaultValue}

Specifically, the grammar for variables is as follows:

variable := '${' variableNames [ ':' defaultValue ] '}'
variableNames := variableName [ ',' variableNames ]
variableName := /* any characters except ',' and ':' and '}'
defaultValue := /* any characters except '}'

Unfortunately, the nature of JSON and JSON Schema make this harder than it appears. JSON allows numbers and boolean values to not be quoted, so using a property in place of such values requires using a string value, and the string value may not be compatible with the allowed JSON Schema type for that field.