Used on the Macintosh to convert a 4-character constant (constant: A value that is not calculated and, therefore, does not change. For example, the number 210, and the text "Quarterly Earnings" are constants. An expression, or a value resulting from an expression, is not a constant.) to a value that may be used by Dir, Kill, Shell, and AppActivate.
The required constant argument consists of 4 characters used to specify a resource type, file type, application signature, or Apple Event, for example, TEXT, OBIN, "XLS5" for Excel files ("XLS8" for Excel 97), Microsoft Word uses "W6BN" ("W8BN" for Word 97), and so on.
MacID is used with Dir and Kill to specify a Macintosh file type. Since the Macintosh does not support * and ? as wildcards, you can use a four-character constant instead to identify groups of files. For example, the following statement returns TEXT type files from the current folder:
MacID is used with Shell and AppActivate to specify an application using the application's unique signature.