When defining HTTP API parameters it is helpful to provide enumerated values when possible. Be as explicit as you can regarding the options that are available to API consumers. Parameter enums help provide guard rails for consumers whenever possible, turning the configuration of API requests into more of a known known, keeping consumers in their comfort zone.
When possible, keep the casing of your enums consistent. You may need exceptions for some standards that may follow a different enum casing strategy, but consistent casing and length of enums for string and numbers help simply the configuration of APIs. Not all parameters will lend themselves to having enumerators, but when it is possible and makes sense, do the work to provide a rich set of options to consumers.