The following white paper gives the history of HED development and the goals for HED-3G
Robbins, K., Truong, D., Jones, A., Callanan, I., & Makeig, S. (2020, August 1). Building FAIR functionality: Annotating event-related imaging data using Hierarchical Event Descriptors (HED). https://doi.org/10.31219/osf.io/5fg73
Robbins, K., Truong, D., Appelhoff, S., Delorme, A., & Makeig, S. (2021, May 7). Capturing the nature of events and event context using Hierarchical Event Descriptors (HED). BioRxiv, 2021.05.06.442841. https://doi.org/10.1101/2021.05.06.442841