What's the difference about sub mode and call activity

i have two process : parent process and child process
i want invoke child in parent
what should i choose? sub mode or call activity

Submodel is an element added by the CUBA platform. It doesn’t create a new process instance it just pastes the submodel as the part of the main model when the model is deployed. In fact there will be just one Activiti process.

Call activity is an element provided by the Activiti framework. It starts a new Activiti process for the subprocess.