With 7.3.0 camunda:out can be used on multi instance call activities, thats why it should also be allowed to use camunda:outputParameter.

AT:

• move notice for camunda:out from master to 7.2 branch of docs [2,3] keep it on master as well
• add Exception when camunda:out is used on MI-Call Activity (master)

[1] https://github.com/camunda/camunda-bpm-platform/blob/master/engine/src/main/java/org/camunda/bpm/engine/impl/bpmn/parser/BpmnParse.java#L3647-3650
[2] http://docs.camunda.org/latest/api-references/bpmn20/#subprocesses-call-activity-passing-variables
[3] http://docs.camunda.org/latest/api-references/bpmn20/#tasks-task-markers-multiple-instance