Question

When would a bill go to a conference committee in Congress?

Answer 1

Going to conference. Conference committees operate after the House and the Senate have passed different versions of a bill.Conference committees exist to draft a compromise bill that both houses can accept. Both houses of Congress must eventually pass identical legislation for the bill to be presented to the President 

Answer 2

After the house and senate have accepted different versions of the bill.  A conference committee is to draft a compromised bill that both  the house and senate can accept.