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When showing infomation about the supplementary comment of revison whose id is inputed into Revision ID input box in Special:RevisionCommentSupplement, "the Supplement Comment" is the supplementary comment or database row of the supplementary comment.
<revcs-show-deletelinktext> is used in $2 of <revcs-show-revision-id>. $2 of <revcs-show-revision-id> contains <revcs-show-deletelinktext>, <revcs-show-editlinktext> and <revcs-show-loglinktext>.
Source code: SpecialRevisionCommentSupplement::getRevisionCommentSupplement - gerrit.wikimedia