Core Sound microphones are offered with two different pickup patterns:
omnidirectional and cardioid ("cardioid" means "heart-shaped").
An omnidirectional ("omni") microphone picks up sounds from all
directions more or less equally. Omni mics typically have a natural
sound. Their transient response is clean and fast, and they provide
very solid bass and crystalline highs. You'd use an omni when you need
to pick up sounds from all around, specifically at the soundhole, in the
interior of an instrument, or at the bridge.
A cardioid microphone picks up better in the direction to which it is
pointed. It tends to reject sounds that come from its rear and reduces
the level of sounds that comes from its sides. When you have a feedback
problem, a cardioid can help. Cardioids tend to have poorer bass
response than omnidirectionals. Cardioids also are much more
susceptible to wind noise. You'd typically use a cardioid mic outside
the instrument from some distance.