How site.tags was generated ?

[Theigrams]

I have a question that how site.tags was generated.

In your blog: Jekyll Tags on Github Pages · Long Qian, which said that site.tags was created by the liquid scripts follows:

{% assign rawtags = "" %}
{% for post in site.posts %}
  {% assign ttags = post.tags | join:'|' | append:'|' %}
  {% assign rawtags = rawtags | append:ttags %}
{% endfor %}
{% assign rawtags = rawtags | split:'|' | sort %}

{% assign site.tags = "" %}
{% for tag in rawtags %}
  {% if tag != "" %}
    {% if tags == "" %}
      {% assign tags = tag | split:'|' %}
    {% endif %}
    {% unless tags contains tag %}
      {% assign tags = tags | join:'|' | append:'|' | append:tag | split:'|' %}
    {% endunless %}
  {% endif %}
{% endfor %}

However, these operations in tags seem to have nothing to do with site.tags.

And how posts are included in tag[1] of site.tags?