A label begins with a letter, followed by an arbitrary number of letters, digits, periods (.), underscores (_). Other special characters can be used in a label, but the label must then be enclosed in single or double quotes. It makes no difference which type of quotes is used, as long as the same are used at either end. The preferred form is double quotes. The use of non-numeric characters is however strongly discouraged, since it makes it difficult to subsequently process a MAD output with another program.
When a name is not quoted, it is converted to upper case; the resulting name must be unique. An identifier can also be generated from a string expression. Abbreviations are no longer allowed as from Version 9.01.