The rank or rating of the crews were not uniform across the Merchant Navy and different Companies called Officers and Ratings by different titles. It is different from the Royal Navy and is quite legal. The companies hire them and call them what they choose. I sailed with PanOcean which had a totally different naming structure to anyone else. These acronyms are right throughout the structure of the MN.
I have been on in excess of 1000 ships in my time and plowed through more thousands of files and I dont know all the acronyms.
When I was a Second Engineer the ships were multi national and BS was a British Seaman, against a Lascar Seaman or a Chinese Seaman. I sailed with British, Indian, Chinese ,Pakistan, Filipino, Malay, Cape Coloured, Italian, Greek.
One thing I learned was nobody knows it all.
regards
jimmy