<set> and <append>

My first question on the description document 
is the difference between <set> and 
<append>.

 

<set name=”items”>
 <xpath 
select=”//mods”/>
 </set>
"Should the response document have 
contained ten MODS records, then after after the response has been parsed the 
“items” variable will contain ten values, each value being a single MODS 
record."

 

This is essentially telling the client that it can 
extract mods records from a response via the supplied xpath.  

 

 

What does this tell the client:

 

<append name=”items”>
 <xpath 
select=”//mods”/>
 </set>

 

Thanks. 

 

--Ray