I was curious and installed the Engineer mode test tool app. It listed 'codes' and for most of them it wasn't written what they do. I couldn't resist stupidity and entered a few of them.
After entering one of the codes (listed under Google Service Framework in the app) my phone rebooted and the Android icon was shown and it said "erasing". I had a mini heart attack because I thought I had reset my phone. A white screen showed up saying something like "Enter password to unlock device". It said I have 30 attempts so I tried entering every password that I could remember but nothing worked. The counter said that I used just one guess no matter how many passwords I entered.
In the end I had to factory reset my phone because I needed it, but my data was on the SD card; as it was adopted I lost all my data.
What have I done by entering that code?
What was that screen that I saw?