Which characteristics work well in a dichotomous key?

A dichotomous key usually uses an organisms physical characteristics, such as:
-Certain limbs (Wings, legs, horns, etc.)
-Certain skins. (Scales, feathers, fur, etc.)
and other things.
That's the best I can describe it, so I hope it helps.

Neurotransmitters are messengers responsible for communication between neurons. they are released from synaptic vesicles and are received by receptors.  the communication is referred to as neurotransmission. the following statement is true about neurotransmitters: a. they're stored in synaptic vesicles in the axon.