Certain pre-processors can be specified globally anywhere in the template that run as soon as the template is loaded to achieve things like random ids generated for each template run.

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randstr

Generates a random ID for a template on each nuclei run. This can be used anywhere in the template and will always contain the same value. randstr can be suffixed by a number, and new random ids will be created for those names too. Ex. {{randstr_1}} which will remain same across the template.randstr is also supported within matchers and can be used to match the inputs.
For example:- 
http:
  - method: POST
    path:
      - "{{BaseURL}}/level1/application/"
    headers:
      cmd: echo '{{randstr}}'

    matchers:
      - type: word
        words:
          - '{{randstr}}'