Partition Types
In this situation, you can use the space allocated to the unused partition. You first must delete the partition and then create the appropriate Linux partition(s) in its place. You can delete the unused partition and manually create new partitions during the installation process.
Create new partition(s)
This step is crucial. Without it, the location of the data could prevent the partition from being resized to the extent desired. Note also that, for one reason or another, some data cannot be moved. If this is the case (and it severely restricts the size of your new partition(s)), you may be forced to destructively repartition your disk.
It is important to understand what the resizing software you use does with the newly freed space, so that you can take the appropriate steps. In the case illustrated here, it would be best to delete the new DOS partition and create the appropriate Linux partition(s).
Create new partition(s)