Do you know if this are examples of non-identifying or identifying relationships?
Is the user_id necessary for identifying the conference? Without user_id, would you still be able to unambiguously identify one conference from another? 

My guess would be yes because I imagine venue_id (which joins to a table of conference centers) and venue_date uniquely identify conferences. If so, then you have a non-identifying relationship between the conferences table and the users table.

For example: in a baseball DB, is team/player an identifying or non-identifying relationship? It may seem that a team would be a necessary component for identifying a player, but think for a moment: what if a player is a free agent between seasons? Then he may not belong to a team for quite a while.

This takes some time to fully contemplate. Try these explanations:

From - https://stackoverflow.com/questions/762937/whats-the-difference-between-identifying-and-non-identifying-relationships


  An identifying relationship is when the existence of a row in a child
  table depends on a row in a parent table....Formally, the "right" way to do this is to make the foreign key part of the child's primary key. But the logical relationship is that the child cannot exist without the parent.


and


  A non-identifying relationship is when the primary key attributes of
  the parent must not become primary key attributes of the child


Here is another example I like: https://www.datanamic.com/support/relationshiptypes.html