During authorization, a parameter appears that is not taken into account in the signature. Why?

Question

User identification

Sometimes, when authorizing an application through classmates, a parameter comes up that is not taken into account when calculating a digital signature. The parameter is called __dp=y
It is not documented.
For example:
https://ok.imper.online/ok/sa-integration?sig=AAAA...
If we read the signature according to all the rules (sort all parameters by keys and concatenate them), then it does not match the one passed. If you discard this parameter - it matches.
Question - where does this parameter come from, why does it appear and how to correctly calculate the signature in the general case (perhaps this parameter is not the only one)
Regards
Roman