Every national association sets two registration periods with FIFA: an international window governing transfers in and out of the country, and a domestic window for moves between clubs in the same association. They usually run together, but they can close at different times, so the two deadlines are not always the same hour.
The distinction is administrative and matters enormously in practice. A transfer from abroad needs an International Transfer Certificate passed between the two associations through FIFA's system, and that clearance has to arrive before the international window shuts, whatever the domestic deadline says.
Because associations set their own dates, a club can find itself able to sign from one country after it can no longer sign from another. The gaps are usually small, but on deadline day small gaps decide deals.
Players under 18 face a further layer. FIFA restricts international transfers of minors to a short list of defined exceptions, which is why a promising teenager can be signed within a country far more easily than across a border.