Start a new project and bring load the default Kevin. Now, under Content Manager > Animation > Expression > Expression_Loop, apply any of the expressions there, such as "01_Anger_Lo". This will immediately show a warning dialog reading:

"The iTalk file contains custom expressions which can not be found on the target character. The result may not be the same as the original."

You will be prompted to press a button labeled "Traditional" or "ExPlus". (The warning doesn't tell you what those buttons will actually do, by the way. Will it modify the iTalk file? Will it modify the character's facial profiles?)

There appears to be no way of avoiding this warning, despite these being built-in content that Reallusion provides. The warning doesn't explain WHICH custom expression can't be found, so there's no way to know how to correct the model. I opened the iTalk file within iClone, to see what expressions it was modifying. I found that there were four "Custom" expressions on the iTalk file, for some reason. (See attached Exp1.png). So I edited the facial profile for the character, and added exactly those expressions. (See Exp2.png). I double checked that EVER expressions I could see on the iTalk file was on the character's facial profile. However, despite all of that, the warning still appears when applying those iTalk files to the character.

So, some basic problems here:
1. Built-in content provided by Reallusion shouldn't produce warnings when used. It's very confusing to user, who are likely to assume they've done something wrong when setting up their character.
2. Built-in content shouldn't have "Custom" expressions in it, unless there's a built-in facial profile containing those custom expressions. Built-in content should be aligned with the built-in Facial Profiles.
3. The warning isn't descriptive. It doesn't explain which expressions are the issue, or how to correct it. It also doesn't explain what pression any of those buttons will do.
4. The warning appears every time you apply the iTalk file, with no default option, no "Stop asking me" option, or anything. Given that it doesn't seem possible to avoid triggering this warning, having it pop up every time is frustrating.