content-lengthwhich tells the recipient how much body it should expect before finalizing processing the entire message and preparing response. When working with web APIs you almost never set this header manually. However, the other very common header
content-typealmost always has to be set by you. This header tells the recipient what is the format of the incoming data. It can be a JSON, XML, binary data of any form, any many more. You can also define own set of headers that are only used by your particular API if that's really needed. However, the standard set of the headers usually is enough for majority of cases.