Semi-truck drivers are in massive demand in the US, and finding a job often means getting some training at a school with Class A CDL driver training courses that include practical skills and classroom training. Typically these schools give you the education and practice you need to ensure you get a license and can get a job driving with a larger trucking company when you finish, but there may be more training to be completed after you leave school.
When you are hired to work for a trucking company, the company will want to see what training you had when you attended a Class A CDL driver training program, and then they may want to augment that with on-the-job training for a while when you first start. Sometimes, that means partnering you up with an experienced driver who can help you acclimate to the job and learn all the essential details you didn't learn in school.
You will learn things like how to work with clients when loading and unloading, what you need to know about weight or inspection station procedures, how to deal with emergencies on the road, and many other things that can be unique to the company you are driving for. The Class A CDL driver training you have is relevant, but it can't cover all the different things that each company requires and does differently from other companies.
Often, semi-truck drivers who have completed a Class A CDL driver training program will start with the basic license and training to get started in the industry. If you want to add additional endorsements to your license that allows you to haul more freight, you may need to attend additional classes that train you for those loads.
After you have the additional training, you will need to take a test at the DMV that shows you know the information required for the endorsement before you are issued the additional rating. Some endorsements you may want include hazardous material, liquid fuels and chemicals, and oversize or overweight loads.
Each state will set the requirements for the additional endorsement to your Class A license, but most try and keep the requirements and rules similar and align them with federal DOT standards to ensure that a driver can haul cross country with the rating they carry. A driver can get many additional load types and special certifications on top of the class A CDL driver training they start with. As you advance and gain experience, these endorsements will often mean finding loads and jobs that pay more and can be more challenging to haul.