This field only works if the Player skull type is selected. In the SkullOwner field you can type in a player's name or paste in a give command generated from http://minecraft-heads.com/custom/heads-generator. MCStacker will extract what it needs from it.

For best results, you should enter a specific player's name into minecraft-heads.com and use the output from it instead of entering only a player's name directly into MCStacker.

Below are some heads you can use: MHF_Blaze, MHF_CaveSpider, MHF_Chicken, MHF_Cow, MHF_Enderman, MHF_Ghast, MHF_Golem, MHF_Herobrine, MHF_LavaSlime, MHF_MushroomCow, MHF_Ocelot, MHF_Pig, MHF_PigZombie, MHF_Sheep, MHF_Slime, MHF_Spider, MHF_Squid, MHF_Villager, MHF_Cactus, MHF_Chest, MHF_Melon, MHF_OakLog, MHF_Pumpkin, MHF_TNT, MHF_TNT2, MHF_ArrowUp, MHF_ArrowDown, MHF_ArrowLeft, MHF_ArrowRight, MHF_Exclamation, MHF_Question.