The arbiter is a high cost offensive-support psionic unit. It relies allied escorts, a fairly high shield and hit point strength to survive the front-lines, rather than its low powered direct attack.
The cloaking field passive ability cloaks all nearby friendly units, but not itself. A popular strategy is to attach arbiters and observers to a carrier/corsair group. The carriers/corsairs attack from under the cloak, and ward off attacks on the uncloaked arbiters.
The stasis ability can cut an enemy attack wave in half, allowing allied forces to eliminate foes in smaller, more manageable groups. A popular strategy is to use stasis to disable high threat units, like siege-tanks or devourers, while destroying weaker enemies.
The recall ability offers a powerful means of strategic movement. Arbiters may use recall to teleport allied forces into an enemy base, circumventing any defenses.