In this video https://www.youtube.com/watch?v=kgF_3FFpixQ at 0:25, the person in the video says "He's been in the army for eight years" referring to his cousin. Does that mean that his cousin was still in the military at the moment he said that, or does it mean that his cousin had been in the military before and he is not in military anymore? Or is it ambiguous?
"He has been in the army for eight years" means he currently is, and has been.
"He was in the army for eight years" means he currently is not, but was for the period mentioned.
"He had been in the army for eight years" refers to a specific point in time in the past used as a reference point, and that he was still in the army at that point and had been for 8 years at the referenced point in time. This is useful for storytelling.