The New Testament of the Orthodox Study Bible is the New King James Version (NKJV). So when reading the New Testament you will often see footnotes at the bottom of the page comparing different historical Greek and Hebrew manuscripts. The most common abbreviations found in these notes include:
NU-Text (or NU): Stands for the Nestle-Aland / United Bible Societies Greek New Testament text. This represents the "Critical Text," which is primarily based on early Alexandrian (Egyptian) manuscripts.
M-Text (or M): Stands for the Majority Text. This represents readings supported by the vast majority of surviving Greek manuscripts (largely Byzantine in origin).
TR: Stands for the Textus Receptus (Received Text). This is the Greek text compiled during the Reformation era that served as the primary basis for the original 1611 King James Version New Testament.
MS / MSS: Stands for Manuscript (singular) or Manuscripts (plural).
Vulg / VG: Stands for the Latin Vulgate, an early Latin translation of the Bible.