From Final Fantasy Wiki

Sacrifice is a spell available only to White Mages in Final Fantasy XI. It must be cast on a party member, and when done, it will transfer one negative status ailment from the target to the White mage.

Sacrifice is augmented by Afflatus Solace. While active, a White mage can absorb up to seven negative status ailments. Afflatus Solace also increases the effectiveness of this spell by increasing the type of status effects that can be absorbed. Effects that will prohibit a White mage from casting though (such as Silence or petrification) will not be absorbed.

Sacrifice does not work on negative songs such as elegy or requiem. It does not matter if the the White mage is currently effected by the same ailment (IE: both the Paladin and the White mage are effected by frost. Sacrifice could still remove Frost from the Paladin).

Rather contrary to its name, Sacrifice will not absorb Doom.

Sacrifice is not affected by Divine Veil in any way.

Sacrifice can be helpful in using Esuna, since Esuna requires the White mage to be effected by the status ailment in order to heal party members of the same ailment.