It is possible to automatically submit a few frames of an agenda-based job at a higher priority than the rest of the frames. These frames are called "preview frames" and may be used to quickly check the validity of a render job.
To submit a Preview Frames job, check the Use Preview Frames checkbox. You may also explicitly specify which frames are the preview frames by explicitly listing them in the Frame Numbers box, using the same syntax used to specify the "Range" (i.e., comma-separated values, such as "1,10,25" or "1-11x5"). If you don't, the system will pick the first, last, and middle frames (3 total) automatically. Note that there may be an upper-limit on how many preview frames you may specify for each job, set by the site-administrator.
You may also specify the priority of the Preview Frames, in the Preview Priority box. Otherwise, the system-wide default (set by the site-admin, or 1 by default) will be used.
The number of subjobs used to process the Preview Frames may also be specified in the Preview Subjobs box. By default, this is equal to the number of preview frames.
Note that when you submit a job with preview frames enabled, it will actually submit 2 jobs—one with the preview frames list at a higher priority, and another with the rest of the agenda, at the normal priority (as specified in the job's Priority field). You will get, consequently, 2 job IDs for the submission.