A neighboring tone is used to
embellish or emphasize a single tone. It may appear above or below the main tone and can be
diatonic or
chromatic (so long as chromaticism doesn't break your
counterpoint rules).
When dealing with chromatics, it is important to remember in your
composition and
orchestration that a chromtic leading tone is more colorful than a diatonic one and tends to draw more focus to the tone it is embellishing.
It is also possible to form a sort of pseudo-leading tone to whatever you want to
emphasize my placing the neighboring tone below the main tone.