Basically the modifiers tell PBP what to expect.
With out a modifier PBP will treat the input as a Display Key:
A2 will be broke into 65 and 50.
Code:
HSERIN [X] 
HSEROUT [DEC X,13,10]
The above will, if sent A2 will output to a terminal
Code:
65
50
13
If the BIN modifier is used PBP will do nothing with A2 because A2 is not binary.
If A1 or A0 is received with
HSERIN [BIN X]
Then
HSEROUT [DEC X,13,10]
will return
1 or 0
The A is ignored.