Siemens S5 DO instruction

Hello, I'm new to S5 programming and I cannot understand how DO function works in the case below.

C  DB50

L DW100

DO =DBWA

T=DROS

Where DBWA is B type, DROS type W used as interface for FB.

Note: on FB PIN DBWA is connected a DB 40, CPU used is CPU928-B

Any help on the matter or understanding DO in Step5 is helpful. All the example's I've found are with using DO with DW, never with DB like here.  Thank you