What is PD1A.180720.030 in Android 9.0.0 (PD1A.180720.030)?

https://developers.google.com/android/drivers contains binary files such as

  • Pixel 3 XL Binary Files for Android 9.0.0 (PD1A.180720.030)
  • Pixel 3 XL Binary Files for Android 9.0.0 (PD1A.180720.031)
  • Pixel 3 XL Binary Files for Android 9.0.0 (PQ1A.181105.017.A1)

What is the number in parentheses?

Plus, I have Pixel 3XL and the system version is Android 9.0, which binary do I have to download?