I understand this, but unless we have a good candidate for this metadata on groups then I'd rather keep it more specialized to the Primitive type and specific to the metadata it actually contains now: scale and precision.
I'd rather avoid premature generalization of types when it is not necessary. As it is defined we just never set it for groups and will also ignore it in groups. It also defines fields that are specific to decimal.
That will make the code that uses this easier to understand.