This allowed external applications, even those not based on ACIS, accessing the data stored in such files. The basic information needed to understand the SAT file format, such as the structure of the save file format, how the data is encapsulated, the types of data written, subtypes and references, is available from this document. However the newer version of ACIS use modified format of SAT files whose specification is not publicly available. Thus reading of modern SAT files requires either using native ACIS library or reverse engineering of the format. A save file contains: a three-line header entity records, representing the bulk of the data optionally, a begin history data marker optionally, old entity records needed for history and rollback optionally, an end history data marker an end marker Beginning with ACIS Release 6.3, it is required that the product ID and units be populated for the file header before you can save a SAT file. Version Numbers and ACIS Releases ACIS is currently being developed by Spatial. They maintain the concept of a current version (release) number in ACIS, as well as a save version number. The save version allows one to create a SAT save file that can be read by a previous version of ACIS. Beginning with ACIS Release 4.0, the SAT save file format did not change with minor releases, only with major releases. This allowed applications that are based upon the same major version of ACIS to exchange data without being concerned about the save version. To provide this interoperability in a simple implementation, ACIS save files have contained a symbol that accurately identified the major version number, but not the minor version. This meant that applications created using the same major version of ACIS would produce compatible save files, regardless of their minor versions. This was accomplished by simply not incrementing the internal minor version number between major versions. Beginning with Release 7.0, ACIS started again providing accurate major, minor, and point version numbers. Beginning with Release 2016 1.0 in September, 2015, Spatial updated to Semantic Versioning, and now describes versions by the model year and major, minor and point releases within that model year. To summarize how release numbers and SAT changes are related: Major release: SAT file changes may be made; significant functionality changes likely; may require significant changes to existing applications Minor release: No SAT file changes are made; may provide