Datastage TX(Mercator) is now owned by IBM, and has been incorporated into the Webshpere Suite:

http://ibm.ascential.com/products/websphere_datastage_tx.html

Though the info there is very generic,

I don't know of a 'clear view' on your question, but basically the Mercator product (as it was known) creates binaries that, in conjunction with an execution engine (somewhat like java byte codes and JVM) that are called Maps. When we say 'Mapping' we mean to define relationships between input and output objects, and in conjunction with additional commands (rules) build one or many outputs , from one or many inputs - each of which can be flat files, databases, message queues etc.

So Mapping is basically to defined the rules , to transform from A to B, and create a 'Map' which can then be executed to actually do the job.

EDI (as you probably know) as well as being a generic term for electronic data interchange , there are also specific EDI standards. From Memory there were ones called (I think) Tradacoms and Tradanet as well as others.

What Mercator also supplies are data structures(these are known as Type Trees) that adhere to these standards, so if you need to send data through these networks the input/output to them is already defined, your job would be to map (match the fields) from your system to the appropriate ones in the EDI structures.

There are also similar type trees for Banking Standards (SWIFT For example) , and has importers to define the same for XML schemas and SAP Idocs