Starting elo rating at chess.com I am confused about the starting elo rating of new members at chess.com.
Most of them start at 1200 but some of them have 800, 1000, 1400, 1600 or even 1800. How is it decided what is the initial value of the elo rating of a new member?
You can select your starting rating on the register page by changing the skill level options.

For example, if you select "expert" your starting ratings will be 1800 (at least when I tried it).

So, to answer your question, "How is it decided what is the initial value of the elo rating of a new member?", it is decided by what option the user chooses.