In terminology of VW-group it's any electronic device, which has been carried out some logic function. Sometimes it looks like nonsens - even simple relay can be called ECU.
The coding of Electronic Control Unit used for explain the ECU where it is and what "neighbours" does it have. Very often on modern cars the coding is crossed with adaptation.
Adaptation of the Electronic Control Unit serves to explain to the ECU with whom and how to talk (If we have started to speak allegories) and serves as storehouse of various adjustable parameters. ( part of memory in the Control Unit which can be changed via internal software )
Measured Values Blocks ( MWB ) – various parameters being inbound or calculated. Considering to value of MWB Electronic Control Unit, trying to "control"... Using of MWB allows you ( if you know the basic parameters ) to find out "close to boundary" or temporarily arising ( intermittent, sporadic ) malfunctions and many other useful things at competent use.