In principle, each country may require child seats to pass certain approval tests, which may be different in different countries. In the case of the US, the approval regulations (the tests that the seat must pass before being put up for sale) are different from those in force in Europe.
This means that a “European” seat that has passed the “European standard” cannot be sold in the US, as it has not passed the US standard (unless it has also been “approved” according to the US standard). And the same happens when using the seat: in order to “legally” use a child seat, it must have been approved for sale according to the approval regulations of that country. Otherwise, we could risk being fined by an agent of the corresponding traffic authority.