CrashPlan allows you to tailor unique settings - called a "Backup Set" - for each file selection you have on your device, allowing you to have multiple selections. This allows you to backup different files at different frequencies and with different schedules.
To check your current file selection's Backup Set, open the application and from the home screen click the "Details" button listed beside your device.
You'll now be shown each Backup Set currently active for your device. The default Backup Set is for your file selection you set up when you first installed CrashPlan. To edit the settings of this Backup Set click the cog button to the right of "Manage Files".
From here you can change the settings for the Backup Set such as backup frequency, versions, file exclusions and so on. Remember to click "Done" when you're finished editing these settings.
If you wish to add a new backup set in Windows click "File" at the top menu and then click "Add Backup Set..." For Mac OS X users this option will also be under "File" at the menu on the very top-left of your screen.
From here you can add a new file selection specifying what files will apply and choose the destination for where you want your files to back up to (you will also have the chance to add a new local destination if required). You will need to finish both these settings before you can add the set. Once you're done click "Add Set".
You should now see your new Backup Set listed. You will be able to edit the settings of this new Backup Set in the same way as previously by clicking the cog button. Please do so to make sure you're happy with the schedule and frequency settings. You can also delete a Backup Set if you wish - this option will only show if you have more than one Backup Set as you must have a minimum of one Backup Set at any time.