Rarity

Rarity is a mechanic in Brave Frontier that describes how advanced or special a certain item or unit is. For units there are 8 tiers to rarity, 1-7 Star, and Phantom/Dream/Omni Evolution. For items, their rarity ranking goes from 1 star to 7 stars.

Units
Specific units have their own level caps depending on when they were introduced to the game, and if they were given upgrades after their initial release.

As discussed in the Brave Burst article, lower rarity units (1-5) only have Brave Bursts, rarities 6 and above have Brave Bursts, and finally, rarities 7 and above have Ultimate Brave Bursts. In addition to BB's, Units 7 stars and above have access to Extra Skills, which are extra passive skills for the whole party, unless otherwise specified. Phantom/Dream/Omni units have the Enhancements feature, which allows them to obtain special changes to the unit. They also have the Resonance feature which gives boosts to units of the same element who spark together. In Global and coming soon in Europe, there is a feature called Omni+ which increases the imp cap for units and their SP limit and also unlocks the Elemental Paradigm.

To evolve units, they require specific elemental evolution units (i.e. Elemental Mecha Gods, Elemental Idols, Metal Mimics, etc.). Global and Europe have their own exclusive units to evolve units (i.e. Wonder Mirror Idols, Tones, etc.), with the Omni+ System, you can either use dupes with Geminus Tomes or Elemental Shards with Elementum Tomes.

Units leading up to Phantom/Dream/Omni evolution can only evolve by one rarity (i.e. 1☆ -> 2☆ -> etc.) while when Phantom/Dream/Omni evolution, a stipulation was introduced that 6☆ and 7☆ units can be evolved (Massacre God Belfura -> Glacial Wings Belfura), in the European server, a specific set of exclusive 5☆ can be evolved directly to Dream Evolution from their 5☆ form (i.e. Extremist Robespierre -> Harsh Terror Robespierre).

Items
Unlike units, items' rarity doesn't have much use other than how 'rare' or 'valuable' they are. (I.e. A sphere with 5% boost may be a 1☆, but a sphere with a 30% boost may be a 6☆.)