Most often, you will select Instruments for Tracks or Parts (by using the Instrument fields in the Inspector or the Track list), but virtually everywhere where you can select MIDI Channels and Outputs you will also find an Instrument field. In the Track list in the Arrange window, you can gain space by hiding the Chn and Output columns (provided you have named Instruments for all Channel/Output combinations you use).
Clicking an Instrument field brings up a pop-up menu, listing the defined Instrument Names. Selecting a Name from the list will either open another sub-menu listing the available Extended Names, or select the associated Instrument (if no Extended Names are defined).