MSI X58 Pro-E - Manual (Page 31)

En-21
S/PDIF-Out Connector: JSP1 (for HDMI graphcs card only)
Ths connector s used to connect S/PDIF (Sony & Phlps Dgtal Interconnect Format)
nterface for dgtal audo transmsson to the HDMI graphcs card.
GND
SPDIFO
S/PDIF Bracket (Optonal)