The phrase "unit milestone" refers to an important event or achievement that marks significant progress or completion of a particular stage or task within a larger project or goal. Full definition
Global Leaf sales passed the 200,000 unit milestone in December 2015, and the Tesla Model S, released in June 2012, ranks second with over 107,000 units sold worldwide. (econotimes.com)
But Media Create's comparison is based on the amount of games sold by the time each console reached the 1 million units milestone for Hardware, and the Nintendo Switch reached it much earlier than the Wii U and the PlayStation 4. (perfectly-nintendo.com)
This brings the total to 482 816 units, which means that it will reach the 500 000 units milestone at retail next week. (perfectly-nintendo.com)