I'm not 100% sure, just 99.99%. Caremark is a PBM and not an insurance company. Caremark as of now does not offer Medicare Advantage Plans. That may change with the merger with Aetna. Silverscript is only a Part D Plan. The MEDDADV, is just the PCN number for electronic billing to differentiate it from commercial plans.
The way Medicare Advantage plans work is that you give up regular Medicare and Medicare pays the Advantage plan a fixed amount of money. The plan then absorbs all of the costs of your care. The Medicare Advantage Plan frequently offers options not covered by Medicare A&B. Additionally, with Medicare Advantage Plans you do not have to by a supplemental policy to absorb the 20% not covered by Part B.
The catch is if you try to go back to Medicare and try to by a supplemental plan you will have to pay more if you have per-existing conditions.