This tab allows the creation of individual functions within each section of Units. (See Units Article)
Here you will see a copy of each Unit Type made, under each of these you can select the "+" button and create a new Function within that Unit Type.
You can also set as to whether the newly created function will be replicated across all Unit Types or will be unique to only the Unit Type it was created under.
Once created, you will be able to select this function in Units under the Unit type it was created in.