What else I require to apply for L Type visa?

Are you sure L type is ok for you? If you attend a training session, you should apply for a F type. The invitation letter, your passport and filled application form affixed with a recent passport size photo are ok to apply for a L type one. By the way, you might be asked to provide bank statement and travel insurance (100USD per day).

Thanks. Our supplier of Generator FPT tried to collect authorised letter from Government. But as per their saying invitation letter is more than enough as they are big group of company and doing these works and inviting people from round world. They did not face this type of letter earlier.

Would you please respond.

Then, you should apply for the F type. L type is for tourism or family visit only. The Invitation Letter you have is ok for F application.