Greencard refers to the permanent resident card in the US. It is normally called greencard because it was originally green in color and it later changed to various colors such as white, pink and white before becoming a green color again.
Greencard allows the person to live in the US permanently. In fact, to maintain the greencard, the holder must stay in the US six months of the year. In such cases, the holder’s country of residence would be the US.
However, the USCIS allows greencard holders the flexibility to live outside the US for six months in a year without any additional paperwork. They can stay outside the US for up to a year with additional proof. If they apply for the Re-Entry Permit, they can stay outside the US for up to two years. In such cases, their true country of residence would be outside the US, even though they are technically called the permanent resident of the US.
Many US citizens sponsor the greencard for their parents so that they can come and go whenever they want. However, many of those elderly parents don’t actually want to stay in the US permanently and they are essentially ‘visiting’ the US, similar to how the tourists on B1/B2 visa would be visiting. In other words, their effective country of residence is not really the US.