For each channel, you can select which of the two solvents to deliver. You can set the percentage of solvent B to any value from 0 through 100 %. Solvent A always delivers the remaining volume: 100 - %B.
For each solvent, click the down-arrow and select a calibrated solvent from the drop-down list. You can also enter additional information (for example, about buffers) in the adjacent field.