It works with the DEC modifier because the PC expects to receive ASCII
data, and this modifier changes the raw number into ASCII.

X VAR BYTE

X = 17
HSEROUT [DEC X] ' sends ASCII 1 then 7. The PC termiinal displays 17

The DEC modifier changes 17 to 2 ASCII characters. A 1 and a 7.

The ASCII value for 1 is 49. The ASCII value for 7 is 55. Try this.

X = 17
HSEROUT [DEC X," : ",49,55] ' Try this

Notice how the DEC modifier has done all the work for you. If you send the
raw numbers 49,55 it displays the exact same thing 17.

Now try this.
HSEROUT [DEC X," : ",49,55," : ",1+"0",7+"0"," : ", 1+48,7+48]

How come some numbers don't display when I don't use the DEC modifier?

Look at any ASCII chart. Or print your own on-screen with this;
Code:
 FOR X = 0 to 63
      Hserout [X]
    NEXT X
      Hserout [13,10] ' CR, LF
    
    FOr X = 64 to 127
      Hserout [X]
    Next X
      Hserout [13,10]
    
     FOR X = 128 to 191
      Hserout [X]
    NEXT X
      Hserout [13,10] ' CR, LF
    
    FOr X = 192 to 255
      Hserout [X]
    Next X
      Hserout [13,10]
And to see how various modifiers work in action;
Code:
X = 17
    Hserout ["X unformatted = ",X,13,10]
    Hserout ["X with Idec modifier = ",Idec X,13,10]
    Hserout ["X with Ihex modifier = ",Ihex X,13,10]
    Hserout ["X with Ibin8 modifier = ",Ibin8 X,13,10]
    Hserout ["X with X + 48 = ",X+48,13,10]
The PC will also send data to your PIC in ASCII. I.E. if you type in 17, and hit
enter it sends two ASCII characters. 49 and 55.

With the example below, what ASCII character do you need to send from
your PC to PIC to stop the Waiting message from displaying every 5 seconds?
Code:
serdata VAR BYTE[2]
X VAR BYTE

X = "A" - "0"
    
    Hserin 5000,Waiting,[str serdata\2]
    IF (serdata[0]-"0") = X then
      Hserout ["Got ",serdata[0],13,10]
      Goto Yahoo
    ENDIF
    pause 2000
    goto Main   
    
Waiting:
    Hserout ["Waiting",13,10]
    Goto Main

Yahoo:
    Hserout ["You are now an ASCII master....;o]",13,10]

Finished:
    Goto Finished
A little experimentation goes a long way....;o]