When using Coverage to represent a Coverage Extension, there's a need to capture a free-text description of the extended coverage - it won't necessarily always be coded.

(In general, all elements in the financial domain that are coded should probably be CodeableConcept rather than coding as there are lots of use-cases for electronic submission of data that will be human-processed. Coding is reserved for those situations where free text absolutely cannot apply and where there's never any possibility of multiple codings being present.)