CALL FOR COMMENTS
A root-level user will specify acceptable values for each field in the cron job.
So if you wanted to limit cron jobs to being executed every 15 minutes, you could specify 15,30,45 as valid values for that field in WHM. Then the cPanel interface could detect the server is limiting that field to specific values, and display a drop-down box with those values. For a field where you specify only 1 valid value, that value will always be used and will just be displayed (rather than a drop-down, effectively disabling it from being changed). This will allow system administrators to do creative things like specify "59" as the only valid value for minutes, forcing cron jobs to rotate around the clock so not everyone's cronjob is executing at the top of the hour.
This settings would always apply, even if a cron job is being made for just a single day of the week.
Original thread: http://forums.cpanel.net/f145/cron-limits-case-54937-a-167230.html