In brief: Canonicalization selects a preferred URL from several similar variants. The rel="canonical" element is a signal for search engines, not an instruction or a replacement for proper links and redirects.
How I use URL Canonicalization (Canonical Tag) in practice
I address URL Canonicalization (Canonical Tag) from the simplest test to more costly diagnostics. First, I try to reproduce the problem, separate a content error from a template error, and determine how many addresses it affects. I deploy the fix to a small sample, check the status and internal links, and only then roll it out more broadly.
What to watch out for
A technical solution must not conceal weak content. URL Canonicalization (Canonical Tag) can remove an obstacle, but it cannot create a reason for someone to visit or trust a page. I therefore assess the fix together with the URL's role, internal linking, and the next step.
Questions for decision-making
- What exactly does the server return, and what does a person see on the important URL?
- Is this an isolated error or a pattern in a shared template?
- What is the impact on discoverability, usage, or conversion?
- How will we test the change before a site-wide rollout?