The defaultNumericFormat build descriptor option controls the initial runtime value of the strLib.defaultNumericFormat system variable, which contains one of the masks that can be used to create the string returned by the strLib.formatNumber system function.
When you are generating a COBOL program, if you do not specify the defaultNumericFormat build descriptor option, the strLib.defaultNumericFormat system variable does not have a default value.
When you are generating Java™ code, the defaultNumericFormat build descriptor option specifies the generated value for the vgj.default.numericFormat Java runtime property (if you have set the genProperties build descriptor option to GLOBAL or PROGRAM). That property then sets the initial runtime value of the strLib.defaultNumericFormat system variable.
You can find more information on system variables and system functions in the EGL Language Reference.