Simplified Technical English was originally developed for use in aviation maintenance manuals, but has expanded beyond this use into a variety of technical fields. It is a "separate" controlled language consisting of a highly restricted subset of simple English words. Are these words are derived from American English, British English, both, or neither?
This is explained in ASD-STE 100 (issue 7, January 2017):
STE approved meanings are based on American English (Merriam-Webster's dictionary).
Use American English spelling.