Spiritual Monsters can be Special Summoned by fulfilling its conditions. Unless its condition requires specific Link Spell, it can be Special Summoned from Extra Deck to Extra Monster Zone or Linked Zones.
Link Spell is a new type of Spell Card. It has 2 effects, first box (top) is Link Effects, second (bottom) is Link Equip effect.
Firstly, any zones that is linked by the Linked Spell can be used to Special Summon Extra Deck monsters. However, most Link Spells have restrictions. For example, the Link Effect of this card states that it only allow the Special Summoning of Zombie-Type "Captain" Spiritual monsters from the Extra Deck.
As Link Spell, it can be attacked and destroyed by battle. It cannot be targeted by an attack IF there is sufficient monsters on its link zone. For example, this card requires GUARD 1, meaning at least 1 monster on the link zone to prevent this card from being attacked.
Next is the Link Equip effect. While face-up on the S/T zones, Link Spell can be equipped directly to monster in its link zone. If you do, attach the Link Spell directly to the bottom of the monster like XYZ Material. While it is equipped to a monster, it loses its Link Effects. It does not occupy S/T zone. It works just like equip spell card except it is not destroyed if the equipped monster is flipped face down, instead remain face up but loses it effects until the equipped monster flipped face up.
As Link Equip Spell, it can still be destroyed by MST and battled. If the card requires GUARD 1, then the Equip Spell cannot be targeted by an attack. However, if the card requires GUARD 2 or above, then it can be attacked and destroyed.