Wikipedia
SAF-T (Standard Audit File for Tax) is an international standard for electronic exchange of reliable accounting data from organizations to a national tax authority or external auditors. The standard is defined by the Organisation for Economic Co-operation and Development (OECD). The file requirements are expressed using XML, but the OECD does not impose any particular file format, recommending that (para 6.28) "It is entirely a matter for revenue bodies to develop their policies for implementation of SAF-T, including its representation in XML. However, revenue bodies should consider data formats that permit audit automation today while minimising potential costs to all stakeholders when moving to new global open standards for business and financial data such as XBRL, and XBRL- GL in particular."
The standard is now increasingly adopted within European countries as a means to file tax returns electronically.
The standard was adopted in 2008 by Portugal and has since spread to other European countries, e.g. Luxembourg, Austria, Germany and France.
Although SAF-T is formally standardized, both with respect to syntax (format) and semantics (meaning) to allow for and fulfill automatic data interchange and tools support, e.g. across country borders or common computerized systems, it do include some room for revenue bodies (tax administrations) to add individual elements, e.g. to cover special needs in a taxation or audit system. For example, in Portugal the SAF-T (PT) v1.03_01 standard - based on SAF-T v1.0 - includes some special elements and types relevant to the standard in Portugal.