Corrective flexes have underscores in their names. That's the only difference, at least in Source 1.

Thanks for your reply. I defined the corrective flex with underscores. For example in my case it be "lippuckerer_lowerlipfunnler" to have a corrective flex applied when both "lippuckerer" and "lowerlipfunnler" is active. In Blender the corrective shape key works. When I generate the DMX, the corrective shape key is not listed as a controller (similar to how they are not listed in the TF2 DMX morph examples). However, after exporting it to Source 2, the corrective flex is created as a seperate flex controller and not automatically applied. When exporting with "simple" flex controller, the corrective flexes are also not exported correctly. For corrective flexes does the controller have to be advanced and something be adjusted in the DMX file? Thanks for any help!