@variable — Use this style of placeholder for most use-cases. Special characters in the text will be converted to HTML entities.
%variable — Use this style of placeholder to pass text through drupal_placeholder() which will result in HTML escaped text, then wrapped with tags.
:variable — Use this style of placeholder when substituting the value of an href attribute. Values will be HTML escaped, and filtered for dangerous protocols.
source:https://drupalize.me/blog/201510/how-log-messages-drupal-8
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