A unique mechanic that The Indigo Disk introduced is a nineteenth Pokémon type, Stellar type. It is not a Pokémon type in the traditional sense and is heavily tied with the Terastal phenomenon in Paldea. It is most useful to think of a new Tera type, not a regular Pokémon type.
What is the Stellar type and how does it work?
The Stellar type is a Tera type that Pokémon can have. No Pokémon has the Stellar type as part of its normal typing, such as Dragon/Stellar.
Defensively, when Terastallized into the Stellar type, a Pokémon’s weaknesses will be retained from their original type. This means a Grass-type Pokémon would still be weak to Fire, Flying, Bug, Poison and Ice-type attacks when Terastallized into the Stellar type.
Offensively, a Stellar type Terastallized Pokémon will receive the same-type attack bonus (STAB) of 1.5 times the base power of attacks but this will only activate once per type. So if you use a Grass-type attack it will be boosted the first time, but if you use another Grass-type attack (even if the Pokémon’s normal type is part Grass) it will not receive a STAB bonus.
The only exception to the one-time STAB bonus is when fighting in Tera Raids, where you will always receive the bonus for moves of any type no matter how many times an attack of that type is used.
Stellar type and Tera Blast, Tera Starstorm
When attacking with Tera Blast or Tera Starstorm, which will become Stellar type, it does not hit for super effective damage against any other type. The one note is that Stellar-type Terastallized Pokémon will take additional damage from the two moves since they deal greater damage to Terastallized Pokémon.
Tera Blast and Tera Starstorm are the only two moves that can inflict Stellar-type damage.