I don't know what is function of "F9" key on my Dell xps 15 9510.

F9 has not been assigned by Dell though on some systems F9 is play and pause. A number of users have asked the same question and it seems to be  a question as to why nothing was assigned.

Here's the link for all keyboard features for your model. 

F9 Does nothing as a media key. 

In my case i Pressed F9 and a genie came out. See if yours does the same

I have the same question & problem and wonder why there's no function for my F9 key. I wish I could just choose the function myself, just as it was with my Lenovo X1 Favkey.

Hi,

I'm using a Dell Inspiron 15 3000, I have the same issue.

Surprisingly, I switched to Firefox a month ago, and I had the curiosity a while back but I tried it out today and to my surprise the F9 button actually toggles reader mode on Firefox, Wonder if it works for any other browsers 🤷‍ .

Thank you