Key highlighted in red

I just noticed that when I am in my FontCreator, my lower case t is highlighted red.
It works when using my font. What does it mean being highlighter in red?

Are you running the latest version?

No, I am not running the latest version. I have an older version. I did not want to pay for a new version when the old version runs fine.
I just wanted to know what the read letter meant.

I just purchased the latest version.
We’ll see if the red letter is still there shortly.