In the Data register (Word 5), it is the 2nd byte that provides the information on the opening or closing of the contact (for example B3).
The transmitter has two contact inputs. This byte should be read as a summary of the opening or closing information on the two contacts.
B3 in hexadecimal = 10110011 in binary. Split the binary sequence in two: 1011 / 0011
First half (1011) for contact input 2. Second half (0011) for contact input 1. (Read in reverse)
The only number we need to know the state of the contact is the first one in each block.
SO
Input 2 - 1011 = 1 open contact
Input 1 - 0011 = 0 closed contact
Here, when you have B3, read it like this: 10110011 = Contact #2 open / Contact #1 closed.
To give other examples:
FF hexa = 11111111 binary = contact #2 open / contact #1 open
3B hexa = 00111011 binary = contact #2 closed / contact #1 open
BB hexa = 10111011 binary = contact #2 open / contact #1 open