transacton in accounting document....represents what?

hello gurus,

in the accounting document, one column trs (meaning transaction) can be seen which carries values like MWS in front of the tax data.

but it shows nothing (like ERL, ERS etc ) in front of other relevant entries. plz guide ??

also can anybody tell what (key ) would be written in front of the customer, like the way it is shown in front of a Vendor?

NOTE : if column trs is not seen in FB03 choose the option from the layout

regards

Vishal