The internal job ticket for each job contains a field that determines the priority of the job.
Tasks for jobs with a higher priority are executed first (unless the higher-priority task has no matching execution slot; see scheduling jobs). This can be used to "rush" a particular job through a flow, or to define a high-priority flow (by assigning a higher priority to all jobs in the flow).
A job priority is a signed integer number (in the range that can be represented with 32 bits). A higher number indicates a higher priority. The default priority is zero; relative to the default a priority can be higher (positive numbers) or lower (negative numbers).
A job's priority can be set through one of the properties of the folder flow element and through the scripting API.