The point of advance care planning is to discuss and inform a patient about the rights they have as far as "do not treat" and "do not resuscitate" choices. Additionally, the provider would instruct the patient how to go about getting their wishes put into writing, ensuring the documentation is legal and can be upheld should the need arise. (Keeping in mind that a provider should never offer legal advice unless they are also a licensed attorney).
One of the primary reasons ACP services are done is because, generally speaking, most patients are unaware that there are multiple types of advance directives, each with a different purpose. Living Wills and Durable Medical POA aka Durable POA for Health Care are the biggies - some states require a person have some sort of advance directive. Some states also have mandatory forms to use, versus a document drafted by an attorney. Ideally, a person would have both a Living Will and a Durable Medical POA, at minimum. Additionally, if a person wants to be an organ donor, there is a separate form/document for that. If a person does carry a legally binding document for organ donation, it trumps a "do not treat" order and also forbids the medical proxy from ceasing life support until the organs are harvested.
If the ACP is stemming from a patient's current condition, potentially terminal or actually terminal, you would use the appropriate DX for that condition. If the ACP is not attributable to any specific condition and is being done proactively, that would occur during an AWV and the AWV DX would be appropriate. For non-Medicare patients, this would be a preventive visit code.
Although CMS does not provide specific DX codes, aside from the AWV, it's common sense that a patient wouldn't normally be discussing ACP during an encounter for illnesses that aren't potentially life-threatening. If you are billing for ACP, you need a medically necessary reason for that service; if an otherwise healthy patient comes in with a runny nose and the provider does ACP during that visit, with no other complaints, it would likely be denied for this reason. Because of the volume of possible payable medical
diagnosis codes, there's no way CMS could specify them all.