What do brackets [ ] mean?

I posted a problem a few days or so ago,
describing that I was trying to use:

   <xsl:choose>
     <xsl:when test=". = 'Transaction_Type'">
       Transaction Description
     </xsl:when>
     <xsl:otherwise>
       <xsl:value-of />
     </xsl:otherwise>
   </xsl:choose>

I received an error:
Expected token 'eof' found '='. . -->=<-- 'Transaction_Type'

and I thought it was because of the namespace that I was using:
http://www.w3.org/TR/WD-xsl. (I know this is obsolete but the
current version of software I am using requires me to do so.)

However, a consultant came in and changed
<xsl:when test=". = 'Transaction_Type'">
to
<xsl:when test=".[. $eq$ 'Transaction_Type']">
and this worked!

He just left (he was working an overnight shift), but I am
anxious to know what the [ ] (with the dot inside it) means.
Also, from what I understand a $ sign is used to get the value
of a variable, but how about $eq$?

Thanks in advance,
-alex
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