they are ...
BDW Block Descriptior Word
RDW Record Description Word
the term sounds a bit funny because only the first halford is used..
both BDW and RDW are usually referred to as having the format "LLbb"
2 bytes for the length and two blank bytes
BDW -- Block Descriptor Word
RDW -- Record Descriptor Word
The length of a variable length record needs to be specified somehow. On the mainframe the RDW holds the length of the variable length record in the first 2 bytes; the last 2 bytes are reserved. The BDW holds the length of the block in the first 2 bytes and again the last 2 bytes are reserved.