[Xiaomi Mi A2 Lite Questions & Answers] GDFI:
For the past years, unlike PC systems, a ROM was device specific and each supported device (of Cyanogenmod, let's say) was maintained by a dedicated developer. We still call it the same version of the said ROM, but each one has specific drivers files for its compatible device. It's still the case nowadays, and that's what you'll see in the ROM section of this device.

However, beginning from Oreo, google introduced a project called "Treble" that intended to separate the common Android System from the device specific files and driver's, by separating the classic system partition to a "vendor" partition, which contains all the hardware specific stuff, and a "system" partition containing a only generic universal Android system.

With this "Treble" project, a new type of ROMs emerged, called GSIs. These ROMs work on all Treble compatible devices (the Mi A2 lite included), and don't need to be maintained for each one. The Treble forum is here ; you'll find a more large selection with GSI ROMs, but I advice you to stay with the device specific ROMs on the Mi A2 Lite forum, which are more optimised for their specific device.

As answered above, these are the specific ROMs especially developed for this device, they'll be the more adapted to the Mi A2 Lite.

UNNOFICIAL just means that the developer that buildt the ROM for this device isn't working directly with the team responsible of the ROM. (For example the CrDroid ROM being official, it's A2Lite version will be hosted in the CrDroid site, an unofficial one won't).

It can be an indicator of stability, but you may find a very stable and clean UNNOFICIAL rom.

Now almost all the ROM that you've seen in the list are based on LineageOS, itself the son of CyanogenMod. We'll come to this point now.

The CyanogenMod project was going on like one week after it was discontinued, under the name of "LineageOS". Its 100% the same source and base, and the LineageOS team is the part of CM team that wanted to continue the CM rom developpement after their team split.

If you want a clean ROM with the basic features and independent from Google software, LOS is the more advisable. They provide their own version of all the basic feature apps (dialer, camera, music player etc) so that you don't need using Google software, and are in general VERY privacy axed, and have a centralised interface called Trust from which you can control what informations each app have access to.

Like I said above, almost all the ROMs (in the A2 Lite section) you'll see are based on LineageOS, adding customization features mostly. So if you just need the vanilla Android feature, with no obscure ROM additional software, and axed on privacy and security, I think LineageOS is your best shot.

The LineageOS 16build for this device is unnoficial but is fully buggless and regularly updated and maintained by a xda recognized developer.

Lastly, unrelated to you questions but you may need to read some "A/B devices - seamless updates" documentation, and carefully read the guides, or you risk bricking your device. (And possibly provide yourself with all the necessary stuff to unbrick your device, just in case)