• For most types of Chinese visa one of the requirements for application is to submit an invitation letter.
• There are a few exceptions, for example the Chinese transit visa (with proof of a connecting flight) and the Chinese tourism visa.
• When you travel to China as a tourist you need an invitation letter if you travel with a travel agency of when you are visiting family or friends. However, if you are an independent traveller you can choose to submit only ticket- and hotel bookings.
• To obtain an invitation letter, have the party or institution you're visiting set one up for you. There is no standard format for the invitation letter, but check the visum requirements for what the letter should contain. For the tourism visa the letter should contain at least information about the applicant, the inviting party and the travel itinary.

For more information about the invitation letter, see: https://www.chinadiscovery.com/chinese-visa/invitation-letter.html