The emergency is a player who would have been seeded withdrawing in the last five days before the deadline. Only player who were ranked high enough to make the original cut-off are considered acceptable substitutes. In most cases there aren't any acceptable substitutes available to play at the last minute because of eligible players already being entered in another tournament or on vacation.
Thanks for all the information, Smucav, I didn't know about this rule
As for Llodra, he was not ranked high enough to be in the original entry-list : if he had applied on time, he would have been second alternate (just behind Serra) but not on the original entry-list.
Anyway, it's a kind of wild card for him, and it means that the organizers preferred him than Sluiter