limit-bytes-in, limit-bytes-out

Hello everybody,

I am trying to set a VPN tunnel with the L2TP of Mikrotik.
In the detailed parameters under ppp secret, there are two parameters, which I am not surre about
limit-bytes-in, limit-bytes-out are set as default values to 0.
I am not sure if this default value is correct.
Could you please help. A link to a description of theses aprameters would be enough.
Thanks in advance for your help
Best Regards
zerces