This is for using the MVD hardware video decoder(unknown whether MVD can do more than decoding). The New_3DS Internet Browser uses mvd:STD for video decoding.
A maximum of 4 sessions can be opened at a time, for all of these services combined.
MVD Service "mvd:STD"
This one uses the I/O mapped @ 0x10207000.
This service is used by the New_3DS Internet Browser.
MVD Service "l2b:u"
This one uses the I/O mapped @ 0x10130000.
MVD Service "l2b2:u"
This one uses the I/O mapped @ 0x10130000.
MVD Service "y2r2:u"
This one uses the I/O mapped @ 0x10102000.