Basically an extra in my book is someone who helps with the roles of extra people. So if I need a few more people in a certain episode, I take the extras I need. Although some may think that they may not be as important as the main characters, but you are just as important. If I get a part I need (like a main character I just made up cause it happens) I could get an extra to fill the role. So anyone can get the role of extra. Good luck!