Main board companies refer to those listed on the main board;
GEM companies refer to companies listed on the GEM. The code name of the securities listed on GEM is four digits beginning with the word "8", which is different from the securities listed on the main board.
For index companies and investors, they may classify companies by industry or company size (such as large, medium and small-cap stocks).
If investors want to refer to the classified list of these companies, they can refer to the index to compile the list of constituent stocks of companies, such as the list of constituent stocks published by Hang Seng Index Services Limited on its website.
I hope I can help you.