Hi Cyril,
not every particular LED can be assigned, but the DOUT registers (group of 8 LEDs).
In the old firmware the DOUT registers were statically assigned to the DINs:
DOUT1 <= DIN2 and MIDI status
DOUT2 <= DIN3 and MIDI status
DOUT3 <= DIN1 and MIDI status
DOUT4 <= DIN4 and MIDI status
DOUT5 <= DIN5 and MIDI status
DOUT6 <= DIN6 and MIDI status
DOUT7 <= DIN7 and MIDI status
DOUT8 <= DIN8 and MIDI status
(“DINx and MIDI status” means: each digitial output is controlled by a button and by the appr. incoming MIDI event which is assigned to that button)
The strange DIN/DOUT mix was necessary, because most people used only three DINs and two DOUTs - DIN1 has some dedicated functions (navigations buttons) which don’t require a LED.
But with the special function buttons and especially the Houston/LC emulation, a more flexible solution was necessary-> the LED maps.
With LED maps you can mix the DIN->DOUT assignments like you want. And it is also possible to assign the DOUTs to other functions:
DOUTx <= MIDI status of DINx only (necessary for the emulation modes, where solely the host application has to control the LEDs)
DOUTx <= internal/external Bank number (in theory, also a LED digit could be driven by one DOUT register)
and planned for MIDIbox64SEQ:
DOUTx <= Sequencer Pos of current track (left/right part)
DOUTx <= Sequencer Pos of track 1/2/3/4 (left/right part)
However, it will be more clear when you are able to play with the different settings. 
Best Regards, Thorsten.
P.S.: from midibox16e.ini
################################################################################
# LED Map: assignes the LED shift registers to the Button Shift
# registers or special values
# Currently following values are supported:
# 0 Default Setting (see Map below)
# 1 Button Shift Register 1 (Button ID #1-#8)
# 2 Button Shift Register 2 (Button ID #9-#16)
# 3 Button Shift Register 3 (F1-F4 and Navigation Buttons: ID #17-#24)
# 4 Button Shift Register 4 (Button ID #25-#32)
# 5 Button Shift Register 5 (Button ID #33-#40)
# 6 Button Shift Register 6 (Button ID #41-#48)
# 7 Button Shift Register 7 (Button ID #49-#56)
# 8 Button Shift Register 8 (Button ID #57-#64)
# 9 Internal Bank (1 of 8)
# 10 External Bank (1 of 8)
# 11-15 reserved
# 16 MIDI Status received for Button ID #1-#8 ([MIDI_LEDS] must be enabled!)
# 17 MIDI Status received for Button ID #9-#16 ([MIDI_LEDS] must be enabled!)
# 18 MIDI Status received for Button ID #17-#24 ([MIDI_LEDS] must be enabled!)
# 19 MIDI Status received for Button ID #25-#32 ([MIDI_LEDS] must be enabled!)
# 20 MIDI Status received for Button ID #33-#40 ([MIDI_LEDS] must be enabled!)
# 21 MIDI Status received for Button ID #41-#48 ([MIDI_LEDS] must be enabled!)
# 22 MIDI Status received for Button ID #49-#56 ([MIDI_LEDS] must be enabled!)
# 23 MIDI Status received for Button ID #57-#64 ([MIDI_LEDS] must be enabled!)
# 24-31 reserved
################################################################################
[LED_MAP]
LED_SR1 = 1 # (Button ID #1-#8)
LED_SR2 = 2 # (Button ID #9-#16)
LED_SR3 = 3 # (F1-F4 and Navigation Buttons: ID #17-#24)
LED_SR4 = 4 # (Button ID #57-#64)
LED_SR5 = 5 # (Button ID #25-#32)
LED_SR6 = 6 # (Button ID #33-#40)
LED_SR7 = 7 # (Button ID #41-#48)
LED_SR8 = 8 # (Button ID #49-#56)
