A subflow can not be used in the windows gui. It's only used in the universal gui or in a system flow. 

@Erwin Ekkel  so it cant be started by a non system flow action?

Correct, in the universal gui subflows can be used.  

Hello Maarten,

A sub flow can be started by non-system flow action, such as a Start task action, as long as it’s from the Universal GUI. The Windows GUI cannot start sub flows in any way.

Sub flows can only contain system flow actions, which is probably where some of this confusion is coming from.

So, in short:

Can a subflow be started with a non system flow task ? (My task is located in the Universal GUI)

Yes

Also if i would place the task in de Win GUI would the subflow still be executed by Indicium?

No

I hope this helps.