The unit section of the library can be accessed by clicking on the "open" dropdown at the upper left of the screen.
This section contains all the units you have access to and can be filtered using the options in the left panel, or searched via the the search bar at the upper right.
Units can be created in this section of the library and accessed in all other parts of Riffyn. The method to create new units in the library is identical to the way they are created in other modes of Riffyn. More information on adding units.
Units you create in the Library are not assigned to a property like they are in design and measure mode). Units are instead available when searching for new units to assign to a property in any mode.