From http://www.bpmn.org/ => Quick Guide => BPMN Basic Rules, I found a basic rule for Sub-Process is that A Start Event in a Sub-Process must be of type None. but right now i can change the start event to other types like message start event or timer in camunda modeler. Do I need to validate bpmn diagrams for this basic rule manually?
You can cange the type of an element via the context pad.
How was the rule phrased? Is it a standards document? Please share with us the original contents of the rule, i.e. via a screenshot of the relevant text.
We enforce proper usage of BPMN on a best effort basis. If this is an actual rule, we should disallow the creation / placement of typed start events inside a sub process.
We already do this, i.e. when moving a non-interrupting start event from an event sub-process to a normal sub-process: