I have been using the schematic of MCB1700 Evaluation board in my customized ARM Powered board. I planned to use RS 232 communication on my board. When I go through the schematic, I am stuck with the #4 pin of DB9 connector which is a DTR Pin for RS 232. My confusion is, that #4 pin is connected to NPN transistor as given in the image. Why is that pin used there? Thanks in advance for the help!

[![enter image description here](https://i.stack.imgur.com/y3QhZ.png)](https://i.stack.imgur.com/y3QhZ.png)