Hello, I'am writing xslt transformation and want to achieve this.
I have this xml:

<Info>
 <CtrlUnit>unit1</CtrlUnit>
 <Freq>1</Freq>
 <Freq>2</Freq>
 <CtrlUnit>unit2</CtrlUnit>
 <Freq>100</Freq>
</Info>

And I want to translate it to this:

<Info>
  <CtrlUnit>unit1<x:br/>1<x:br/>2</CtrlUnit>
  <CtrlUnit>unit2<x:br/>100</CtrlUnit>
</Info>

So to select following Freqs but only to next CtrUnit.
How can I achieve this?

This problem is known as "positional grouping", you will find much discussion if you google for that.

In XSLT 2.0 it's easy,

<xsl:for-each-group select="*" group-starting-with="CtrlUnit">
  <CtrlUnit>
    <xsl:value-of select=".">
    <xsl:for-each select="Freq">
      <x:br/>
      <xsl:value-of select="."/>
    </
  </
</

In 1.0 it's much harder. There are basically two techniques: "sibling recursion" in which you write a recursive template that runs through the sequence of siblings one element at a time, or "Muenchian grouping", in which you reduce the problem to value-based grouping using the Muenchian method, taking generate-id(preceding-sibling::CtrlUnit[1]) as the value of the grouping key.

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference