For racer, there are 20 tiers of missions, each has 3 assignments you can pick. Racer choices are Race, Pursuit, and Drive.
Racers have 4 different types of Missions: Interceptor, Race, Hot Pursuit, and Time Attack.
Interceptor the easiest mode. You want to use Shockwave and Electrostatic Field(ESF) and just wreck the cop, don't run from him.
Races have you just racing other racers, but cops will join in based on heat level. These can get a bit frustrating once you get to the medium and hard difficulty as you are relying a lot of cop interference then you are actual driving performance. Easy you race 3 opponents, medium 4, hard 5.
Hot Pursuit are very similar to racers, except the cops are chasing you from the start of the event.
Time Attack are straight forward events that have you going from point A to B. The times can be very strict so typically if you crash more then 2 or 3 times, you won't get a gold time.
When picking assignments, from easy to hard, you want to pick Interceptor, Time Attack, Hot Pursuit, Races. Also, for the assignments that require you to earn a certain amount of money in a session, find an interceptor near a repair station and do it several times. This also works for assignments that require you to reach a HEAT level.
As you go through the story you can only pick one assignment at each tier of the story, putting you at level 21 when you beat the game. At this point, the game will let you go back and do the other 2 assignments for each tier.
You will receive rank 20 achievement for unlocking the final assignment.
You will receive rank 40 achievement for finishing 40/61 assignments.
You will receive rank 60 achievement for finishing 60/61 assignments.
Extra: For people who are having issues with the save glitch, the best method I have found to work is when you are done playing the game. Go to your home screen, open the start menu over the game and click the quit option. Once I started doing this every time I was done playing, I never had it lose progress.