PQ is a Congestion Management technique. PQ schedules traffic such that the higher-priority queues "always" get serviced first. This can cause the traffic of other lower-priority queues to starve out. PQ uses 4 different queues- High, Medium, Normal and Low.
The SPT orders the jobs based on Congest. The jobs with the smallest processing time are scheduled first, while The jobs with the largest processing time are scheduled last.