The array property ClassIDs defines unique class identifiers which can be used to identify a particular class of particles. The class number is specified by the parameter ClsIx (valid range: 1..NClasses). If you assign an identifier to a non-existent class (i.e. if the ClsIx is less than 1 or greater than NClasses) the call will be ignored on write or return an empty string on read.
Please note the subtle difference of the properties ClassIDs and ClassNames. The ClassIDs are mostly technical unique identifiers, typically used to reference a particular class for programming purposes. The ClassNames property on the other hand should be used to display the name of a class. This way you can change the name of a class without losing any class relationships in your program.