Fiio M15S - Audio Output and Gain Settings; 3.5 mm Output Selection; 4.4 mm Output Mode Selection; SPDIF Output Configuration

3.Audio
3.5Output selection
PO (Power output), generally used when connecting to earphones and
speakers
LO (Line out), generally used when connecting to power amplifiers
SPDIF (Coaxial out), generally used when connecting to DACs