See for example the following bucketset that defines a list of values to be used for some status field:
Anyway, bucketsets are typically used in decision tables to define the set of values to be used in conditions. Unfortunately you cannot use them to define the set of values to be used in actions. At least, that is what the UI seems to suggest, as bucketsets do not appear in the values you can choose from.
But don't get fooled, you are being misguided, as it works after all. Just type it in as [bucketset_name].[value_name] and be surprised that it works!
Funny enough this is not the case for IF-THEN rules.
Let's call it room for improvement.