it confuse me too. not just yobit, many sig campaigns require personal message like that too, include this one, 777coin. but as i research the spreadsheet of 777coin and find out some high rank members use other personal message and still get payment, i wonder if it's allowed or not. if yes, i want to erase my personal message and join message campaign of others. i have asked izanagi about it but he never answer.
The guy was on YOBIT signature campaign which there was a clear rule that PM should be set to yobit.
In your case, 777coin also has a rule that your PM should be 777coin if you are enrolled in their signature campaign.
There is a separate pay for 777coin PM which allows people from other campaign to advertise 777coin in PM.
For example if you are in Bit-X campaign there is no rule there that their PM should be Bit-X so they can participate on 777coin PM campaign.
Did I explained it clear or did I only made it more complicated? LOL.