Hi Steve,
Thank you for response/
According to customer experiment, if they connected the VSSRTC to the board ground with using external crystal, the application which used C5535 showed an improved tolerance for the static electricity. So, if there any problem for this, they will connect the VSSRTC to board ground.
I do not know why VSSRTC must not connect the board ground with an external oscillator. I think that there are some reasons that you described this to datasheet.
Best regards,
Kaka