i will explain it for you since it seems like you cant figure it out on your own.
Paladin: holy warrior infused with light.
Prot: uses light to increase survival.
Ret: Use light to infuse melee attacks and increase melee damage
Holy: Uses the light from themselves to heal.
Priest: Channels light or shadow
Holy: calls upon light to heal allies.
Disc: calls upon light to deal damage and heal allies. sometimes uses shadow
shadow: uses shadow magic to damage.
now for the point.
necromancers are known for 2 things, Summoning undead minions and spread plague.
for the sake of example i will use names for specs.
Bone: Summons undead minions to overwhelm enemies. (issues with this is Unholy Dk and demonology warlock basically so the same thing)
Plague Doctor: spreads diseases to kill enemies. this sounds like a dot style spec. (overlap with Unholy dk and affliction warlock.)
Transfusion: drains the life of enemies and transfers it to an ally. (no over lap here.)