Container Variable enables you to cache the values that you want to read again when you next call the same user-defined function. An instance of the container object is transferred to each user-specific function by using the container parameter.
Go through this help for further information on this:
The "container" in the UDF are similar to that one in BPM, as concept.
But the container in UDF completety indipendent from the BPM's one.
Read the <a href="http://help.sap.com/saphelp_nw04/helpdata/en/75/8e0f8f3b0c2e4ea5f8d8f9faa9461a/frameset.htm">SAP Help for GlobalContainer Object</a> and you will know everything about it.