[SI-LIST] Clock signal termination

Hi all,
In the schematic of an evaluation board I've seen some clock nets like that:


                                                                                
                   parallel term to gnd R=178 ohm
                                                                                
                                  |
                                                                                
                                  |
                                    |---SERIES TERM 36ohm----Tline 55 
ohm------|----RECEIVER
DRIVER 7 ohm--------|
                                    | --SERIES TERM 36ohm-----Tline 55 
ohm------|---RECEIVER
                                                                                
                                   |
                                                                                
                                   |
                                                                                
                      parallel term to gnd R=178 ohm  

Can someone explain me why the value of the parallel term is 178 ohm?
Any comment will be appreciate.


Thanks in advance
Lucas

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