A flat file schema defines the structure of a class of instance messages that use a flat file format, either delimited or positional or some combination thereof. Because the native semantic capabilities of XSD do not accommodate all of the requirements for defining the structure of flat file instance messages—such as the various types of delimiters that might be used for different records and fields within the flat file—BizTalk Server uses the annotation capabilities of XSD to store this extra information within an XSD schema. BizTalk Server defines a rich set of specific annotation tags that can be used to store all of the required additional information.