Let see the picture below to understand the instrument window. This window will allow you to configure the mapping of the instrument's sample. 

Instrument window.

Selecting a sample in the sample list allow you to fix its parameters. Usualy fix the Base parameter (which is the base note for the sample). The base parameter is important because it indicates to Bexpand the default note for the current sample. Playing the base note in your Midi keyboard will play the original sound sample, else Bexpand compute a new sound.

The From... To...  parameter indicate the mapping of the sample in the Midi keyboard. For example, a C2 to C3 mapping indicate you use the current sample when you play note between C2 and C3. Out of this range, you use another sample. So you can put severals samples in an instrument. You can assign one sample per note (so 127 sample per instrument) and more if you use the From vel... to vel....  parameter.