Sometimes in your XSLT you need to access the data not just on the $currentPage but possibly any parent nodes.
For example using the related links data type to ouput a list of links on the currentPage works OK if I have that property on that page
<xsl:for-each select="$currentPage/data [@alias='links']//link>
...
</xsl:for-each>
However if I have this XSLT macro in a masterpage within a sidebar and navigate down to a child page, the above XSLT would not work unless I have that property on that child node. So to solve this I have modified the XPath.
<xsl:for-each select="$currentPage/ancestor-or-self::node/data[@alias='links' !=''][1]//link">
...
</xsl:for-each>
$currentPage/ancestor-or-self::node/data[@alias='links' !=''][1]
What this new XPath is doing is saying walk up the tree until you find a node with a property called links that is not empty, then Select the first item, which would be the lowest item when walking up the tree.