It depends entirely on the context. Can you give an example (preferably usage example) of the kind of constant you're referring to?
I.e. there are constants for default settings (e.g. Settings::DefaultModule or something) , constants referring to values (e.g. $Dave->setTieColour(Colour::Blue) or using binary flags, $Dave->setTieStyle(Ties::Blue | Ties::Stripy)), etc.