In the FAST search system, an item processing service performs required processing on content retrieved to prepare it for indexing. Examples of item processing are mapping of crawled properties to managed properties, parsing of document formats, linguistic normalization of text, and normalization of numeric data.
The item processing service then sends the processed items, in the FAST Index Markup Language (FIXML) format, to the indexing service. The following figure provides a high-level overview of a search system that uses the FIXML data structure.
The purpose of FIXML is to provide the data structure for a processed item and then to map the data to the search index structures that an index schema specifies. The processed managed properties are mapped into the appropriate FIXML data structures based on the mapping configuration, as described in the fixml_mappings.xml configuration file.
The indexing service uses the FIXML data structure to create the index data structures. One instance of the FIXML data structure represents the processed data of one item after item processing.
You can read more about FIXML in the [MS-FSFIXML]: FIXML Data Structure document.