What's the trigger condition of Transfer-Encoding: chunked for Tomcat 8

Question

Here is the configuration in the server.xml in my tomcat 8.

<Connector port="8080" protocol="HTTP/1.1"
           connectionTimeout="20000"
           redirectPort="8443" />

and I just find that some RESTful api requests return with Transfer-Encoding: chunked header, some are not. And looks it's not following the rule that I understood before: "the response that content size exceeds a threshold (say 8k) will return with Transfer-Encoding: chunked"

so my question is that what is the exactly trigger condition of Transfer-Encoding: chunked

Fascista