It is no different for a passport card; however the digital ID system is not going to be accepted for customs use. It can be used for TSA check in, but not for customs.I am wondering how that it is going to work if you already have a physical passport card as I do. I have the passport but I also have the passport card and when I travel to Canada or Mexico I just present the card and I then clear customs without an issue. I have the actual passport with me in my bag if there is further questions but have never been required to produce it. The actual passport card is acceptable for travel to Mexico and Canada
In addition, when you travel, you are told that you must also have valid photo ID on you even if you can use the digital ID system. Of course it’s a fallback in case the reader doesn’t work or your phone died/is lost, but the TSA can also ask you to see your physical ID under the discretion of the officer. It is not correct to travel with just digital ID on you.