One continuous year of qualifying employment with the group outside the United States, completed within the three years immediately before the petition is filed. Time the employee spent working inside the United States for the same employer does not count toward that year, even though it does not necessarily break the employment relationship.
Count backwards from the filing date, and count carefully
The counting is mechanical and the mistakes are always the same. Start from the intended filing date, look back three years, and find twelve unbroken months of employment abroad with a qualifying entity in a qualifying capacity. Employment with a company that turns out not to be part of the group does not count.
Periods spent working in the United States for the same employer are excluded from the twelve months, which surprises people whose job already involves regular southbound travel. Where the year is short, the practical answer is usually to wait rather than to argue. Hypothetical example: an equipment-dealership operations director spent six months of the last two years stationed at an American customer's site for the same employer, and assumes it all counts as service; it does not, and the twelve months have to be found in the periods on either side of that assignment.