Reference:Identifiers

POV-Ray allows you to define identifiers for later use in the scene file. An identifier may be 1 to 40 characters long. It may consist of upper and lower case letters, the digits 0 through 9 or an underscore character ("_"). The first character must be an alphabetic character. The declaration of identifiers is covered later.

All keywords (reserved words) are fully lower case. Therefore it is recommended that your identifiers contain at least one upper case character so it is sure to avoid conflict with reserved words.