XSLT select // and .// - xslt

Question

What are the difference between // .// when we set path?
And the second question if I want to get some information of parent node in the for-each block how I can get it?

Answer 1

// is short for descendant axis.
If you say //para, it selects all para elements in the whole document.
When you say .//para, all the para elements which are descendants of the context node are selected.
For a demonstration, consider this XML:
<l1>
<para>1</para>
<l2>
<para>2</para>
<l3>
<para>3</para>
</l3>
</l2>
</l1>
and this XSLT:
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" indent="yes"/>
<xsl:template match="/l1">
<output>
<xsl:apply-templates select="l2"/>
</output>
</xsl:template>
<xsl:template match="l2">
<para-all>
<xsl:copy-of select="//para"/>
</para-all>
<para-context>
<xsl:copy-of select=".//para"/>
</para-context>
<parent>
<xsl:value-of select="name(parent::*)"/>
</parent>
</xsl:template>
</xsl:stylesheet>
And the output you will get is:
<?xml version="1.0" encoding="utf-8"?>
<output>
<para-all>
<para>1</para>
<para>2</para>
<para>3</para>
</para-all>
<para-context>
<para>2</para>
<para>3</para>
</para-context>
<parent>l1</parent>
</output>
The para-all gets all the paras in the document, whereas para-context gets only those paras which are descendant of l2(the context node)
And about how to select the parent, as in the code(see parent element in XSLT).
In the example above, the context node is l2 and by using parent axis, the parent element's name is being written.
You can use either parent::* or ../