\dif command from commath package is not recognized. Thus, it is marked as unrecognized command in texstudio.
However, the other commands defined in the same package are recognized without any problem. I don't know why.
I know I can suppress this warning by configuring the custom highlighting tap.
I am just wondering why \dif is not recognized.
This is because the .cwl for commath is autogenerated. See the FAQ for more details.
If you chose to create a .cwl for the package yourself, you can send it to us and we'll include it in the next release.
Oh I should have understood how TXS works before I posted this report.
Sorry for the noise. Anyway, now I understand more about it.
As you suggested, I have written a cwl file using the auto-generated cwl file.
According to the document about commath from CTAN, I think there is no missing commands in the attached file.
Let me know if there is something I can help you more.
Thanks. I've added the cwl to the repository.
Last edit: Tim Hoffmann 2014-01-12