AS Autonomous system (Internet) - Wikipedia
You can also find ASN on ipinfo.io
when you give an IP, it shows you which organization it belongs to
and it tells you if it’s ISP or VPN or HOSTING etc. or all together.
when to block ASN?
Mostly when it belongs to some hosting
and when you simply have no choice because a simple IP ban doesn’t help because they keep coming back with a new IP
or there are a large number of IPs and blocking ranges that make it easier for you to put a block on the entire ASN.
What should not normally do is to block the ASN from some ISP
because there are 99% of your real visitors (people) as opposed to bots, VPN and Tor.
there are also several paid VPNs that use residential IPs to bypass the block
but fortunately there are still few of them
why CF does not block ?
probably something is wrong in your settings or is the wrong priority
but it’s a question more for a Cloudflare community forum
Just for the sake of comparison
The biggest ever DDoS attack in world history = 3.47 terabytes per second (UDP reflection attack - 15 minutes)
Cloudflare’s network capacity = 121 Tbps