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.