a) Remove Contexts: - remove all higher level contents of 
the source level message in order to map with the target 
message field. So you can get the node contexts removal.

b) Split by value: - Insert Context change for an element 
split by value is some as counter part of remove context. 
Here instead of delivering the context you can insert a 
context change in the source value queue.
c) Collapse contexts: - Copies first values of the all 
contexts to one context empty context are replaced by empty 
string.
d) Copy Value: - How many ever times I occur I am copied 
just once". This is not a node function but I am explaining 
it as it is used very frequently in real-time. Let us take 
an instance of material group, which can occur as many 
times as the item node. But since we are mapping it to the 
header in the target node I just want to copy the first 
occurrence of material group and map it to the header node. 
CopyValue exactly does that.
e) Create if: - It will create a target node or element 
based on some condition.
f) Exists: - Function to determine whether a particular 
source field exists in instance to be processed. If it does 
it gives true or else false.
g) Use one as many.
h) Replace value: - Replace the value I with the value that 
you desired in the dialogue for function properties.
i) Sort: - Sorts all values of the multiply occurring 
inbound field with in the existing or set context. 
 
   http://sapxifaqs.blogspot.com

UDF are are user defined functions may be simple or advance.
You have to create new UDF at message mapping editor. You 
can create UDF according to your requirement in order to 
fulfill your requirement if it is not avaialble in standard 
functions.