Yes. The idea of PLoS One is that people can subject directly to it, but also it serves as a second chance for papers submitted to the higher tier PLoS journals

Yes. The idea of PLoS One is that people can subject directly to it, but also it serves as a second chance for papers submitted to the higher tier PLoS journals -- an editor can reject the paper from those while still suggesting that the authors submit it to PLoS One (if the reason for rejection was that it wasn't that exciting but still solid research).