What is PD1A.180720.030 in Android 9.0.0 (PD1A.180720.030)? https://developers.google.com/android/drivers has binaries like 


Pixel 3 XL binaries for Android 9.0.0 (PD1A.180720.030)
Pixel 3 XL binaries for Android 9.0.0 (PD1A.180720.031)
Pixel 3 XL binaries for Android 9.0.0 (PQ1A.181105.017.A1)
...


What is the number in the parenthesis? 

Further, I have Pixel 3XL and the system version is Android 9.0, which binary should I download?
They are build numbers:
According to Pixel 3 and Pixel 3 XL First Factory Images are Available!:

Google just posted the first factory image and OTA files for the Pixel 3 and Pixel 3 XL. Since today is basically launch day, the timing seems just about right.
The files are available as Android 9.0.0 builds PD1A.180720.030 and PD1A.180720.031 for both phones, the second being the Verizon specific build. This is the build you should find out of the box.

About PQ1A.181105.017.A1, a reponse is found in this article:

Google is rolling out the November 2018 security patch update for its latest flagships Pixel 3 and Pixel 3XL. The current updates are also making their way to the older devices Pixel 2 and Pixel 2XL.
The new build number which accompanies the software goes by PQ1A.181105.017.A1. Both Pixel 3/3XL and Pixel 2/2XL are getting an upgrade with this new build.

Summary:

PD1A.180720.030: International build (released date: 20 July 2018).
PD1A.180720.031: Verizon build (released date: 20 July 2018).
PQ1A.181105.017.A1: November 2018 security patch (released date: 05 November 2018).