The southeast of a place, country, or region is the part which is in the southeast.
东南部 • Record levels of rainfall fell over the southeast of the country. 该国东南部降下了最高纪录的雨量。
The southeast part of a place, country, or region is the part which is toward the southeast.
东南部的 • ...rural southeast Kansas. …堪萨斯东南部的农村。
If you go southeast, you travel toward the southeast.
朝东南方向 • I know we have to go southeast, more or less. 我知道我们大概得往东南方向走。