Typically, a sound card mixer has a line out control, with the controlled source as the Audio signal provided to the line in jack of a sound card, an aux out control that controls the audio signal provided to the line out jack, and some Mono out / PC speaker control and a SPDIF control.
Line out Aux out Audio signal provided to the LINE OUT jack of a sound card (colored lime green in PC99). This can be connected to headphones or a HiFi amplifier, etc. Audio signal provided to the AUX OUT connection of the mixer. Not all sound cards provide this mixer channel on an external connector. Mono out / PC speaker SPDIF 
