Simply put, to use an equivalent in the US, Saturn would be like a top minor league baseball team, and USPS would be the major leagues. Rider talent, length of races, and cost are all part of it. But the teams can opt for what level they want to compete, and cost and salaries are involved. But if you obvioulsy do not have the talent and results, you cannot jsut show up and opt for Div 1. THEN, you have to put up a bond to secure costs with UCI to ensure you will be around and sometimes teams have to have UCI pay the riders.
The UCI controls all of the "trade teams" that are professional, world-wide as well as the races in which they race. Your results are measured by how you finish and the ranking/rating of the race. So, if you look at the UCI schedule, you will see each race, rated, like 1.1, 2.1, or "Grand Tour". The closer to 1 you are the more dificult the race. So, Mercury, two years ago opted for Division 1 and had hoped their results woud get them into the TdF. The did not and the economic results were devastating.
UCI has a website. The top ten (if my memory is decent) Div 1 teams in the world get into the tour. There are four wildcard teams. The last two are selected in April, I believe. That is how teams like Jean Delatour and Ag2R get into the Tour. Wildcards.