although nerc was responsible for the e-tag spec, it opened development of the application to run it to the software market. initially there were numerous e-tagging software providers, mainly a mix of small start-ups and new applications developed by existing energy industry software developers. the e-tag 1.5 spec was written in such a way that the various applications were permitted to have differing graphical user interfaces (guis), but functionally "under the hood" they were required to be able to interact with each other when transmitting, receiving and processing e-tags. a new feature introduced with e-tag 1.4/1.5, made possible by the real-time sharing of e-tags, was the ability for reliability entities (namely the ca's and tp's) in the e-tag to electronically approve or deny e-tags based on various criteria.