Paul Richmond, Alex Cope, Dave Allerton and Kevin Gurney wished to implement a simulator toolchain for models encoded in NineML. They found that some details were missing from the NineML specification, preventing them from implementing their models. Taking a pragmatic approach, they made necessary additions to the NineML spec to suit their needs. It is hoped that in the future, SpineML will become an officially recognised extension to NineML.
You can read about SpineML in the following paper:
Richmond P, Cope A, Gurney K, Allerton DJ. “From Model Specification to Simulation of Biologically Constrained Networks of Spiking Neurons” Neuroinformatics. 2013 Nov 20
For the full detail, refer to the SpineML schemas.