Thank you for your feedback. Setting a rule to camel_Underscore would allow words that are camel case to have underscores within them. For example camel_Case is allowed from this rule. However, camelCase words start always with a lower case letter. That is why a warning is shown when an identifier like this is found _camelCase and the prefix box is empty.
As you have found out - if you want identifiers to start with a underscore, you should add underscore in the prefix box. If you want them to contain underscores set the rule to camel_Underscore.
If you have any more questions or remarks, please do not hesitate to contact us again!
the Telerik team
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