This brake Line T is for 10mm 1.0 pitch brake lines with the flare type end.
If you're installing a dedicated hydro with secondary calipers or just want to redo those old brake lines, this T is a good way to split the hard lines. Made from plated steel, the threads will hold up well without a big fear of stripping when tightened. The T also comes with a mounting hole to attach it to brackets.
This is common on Nissan and Toyota and other Japanese imports.