The SIF Infrastructure Specification 2.7 is the culmination of the 15 years of development that went into the previous SIF releases. 2.7 is the oldest version to still be actively supported by the SIF Certification Program.
>> SIF Data Model Implementation Specification (North America) 2.7M
>> SIF Data Model Implementation Specification (Australia) 1.4
The only change between the SIF Infrastructure Implementation Specification 2.6 and SIF Infrastructure Implementation Specification 2.7 is a change in the version number. This is important so that data model expectations can be managed over the wire.