Custom Choose from all of the available applications. After selecting the type of installation, click next to continue. Do not select this option unless all of your important information is saved on other media. If you choose this option and click next , a warning box will appear asking for a confirmation. Click yes to continue or no to cancel and select another destination.
Unused space refers to unpartitioned hard drive space.
After selecting this option, click next . The following dialog box will allow you to select the device and set the partition size. It will allow you to Add and Delete partition information. Make sure you have a backup of all important data before choosing this option. After selecting this, click next .