Let me add some additional information on this subject. As Pedro mentioned, Software Units are calculated by counting all elements of a given application, weighted by the associated underlying complexity. For those of interested in the details of the elements and their weights, you can check the attached document. It contains the weights applied to each type of element, as available in Service Studio.
Bear in mind also that you can check the Software Units consumption per eSpace in your Service Center. Logon to Service Center, go to "Administration" tab, "Licensing" sub-tab, and click the "Software Units" link. There you'll find the exact use of Software Units per eSpace.
Hope it helps better understanding the concept.
As a general principle, base eSpaces such as Enterprise, RichWidgets are WidgetLibrary are provided as is in open software so that everyone is able to customize them for specific needs.
For those eSpaces, the original implementation is redeemed from Software Unit consumption. So, in order to allow everyone to customize them, the Platform allows up to 10% additional Software Units as customization. When such limit is reached, the excess Software Units will be accounted for and presented in Service Center accordingly.
Let's imagine EntepriseManager has 100 SUs. Then, you can customize and extend it up to 110 SUs (+10% of original) without having those SUs accounting for consumption in your license.
Once you customize or extend EnterpriseManager up to 111 SUs, the 1 extra SU will consume 1 SU from your license. If you customize it up to 200 SUs, then 90 SUs will be consumed from your license (200 - (100 + 100 * 10%)).