The Public Suffix List (PSL) is a community-maintained list of domain suffixes that helps software determine where registrable domain boundaries fall. For example, the public suffix may be more than one label long, so simply taking the last two parts of a hostname is not always correct.
Why crawlers and data tools use it
The PSL can help group URLs by registrable domain, apply per-site limits, or reason about cookie scope. Use a maintained library and current list rather than writing a simplistic string rule. The list changes as domain registrations and policies evolve.
The PSL is not a DNS database and does not tell you whether a host exists or who owns it. It describes suffix boundaries for software use. If domain grouping affects request pacing or data joins, record the library and list version used so results can be reproduced.
Avoid treating subdomains as interchangeable by default; they can be operated by different services or have different access policies even when they share a registrable domain.
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