The top priority is given 12 points, down to the twelfth which gets one point, and then the order of business at Conference is decided by the Standing Orders Committee broadly in line with these preferences. Motions lower down the order of business are available for “reprioritisation” for discussion on Friday afternoon (subject to the Conference not being closed early in error of course!) The reprioritisation process is one of the most democratic features of the Conference (which is why many people in UNISON devoted considerable energy in a doomed attempt to do away with it and are now trying to squeeze it out of existence by timetabling other business on Friday afternoon).
Motions which attract no support from anywhere in the prioritisation process are not available for reprioritisation and so will not be debated (the NEC will determine UNISON policy on any that are not withdrawn after Conference). Just because a motion gets some support in the prioritisation process that is no guarantee that it will be debated (which is why there is a reprioritisation process) – however missing out at this stage ruins the chances that a motion can be debated at all