As the short guide to BiCon jargon says, the DMP is the 'Decision Making Plenary', a session where people at BiCon debate things relating to BiCon and the wider UK bi community. Votes are taken for some things, about the only time at BiCon that that happens, and it is possibly the nearest thing the UK bi community as a whole comes to making decisions.
2003 (where it was the 'Closing Ceremony').
2016 (which called it the 'Decision-Making Meeting').
2017 (which also called it the 'Decision-Making Meeting').
More will appear here at some point.