What is the IoU in the configuration file of Tensorflow object detection API

[Lyhour-Chhay]

Excuse me everyone, in the configuration file for training the model faster RCNN in tensorflow object detection API, there are two IoU: the first stage of IoU is 0.7 and second stage post processing IoU 0.6. I understand what the IoU meaning is. However, I want to know basic of these two IoU in the configuration model faster RCNN. I think the first IoU 0.7 use to get label and for the second IoU what is use for? I don't know about the second IoU. Thank you very much.