Ok, I have just figured out. While in the IF-AN24 (Analog I/O) and IF-TD24 (TDIF) the led is green, in the IF-AD24 (ADAT) and IF-AE24 (AES/EBU) the led is orange.
It's on each card's pcb. On the analog i/o card is labelled as 'Clock OK' and on the digital cards as 'Data'. I asked because while the led is green on my TDIF card, it's orange on the AES/EBU and ADAT cards. Anyway if there is a reason for that difference, I guess that will be related with the revisions of each card.
Cheers