The rarity tiers
Blue Prince rooms carry a rarity label. Across the 110 rooms in the database the values include N/A, Unusual, Rare, Commonplace, Standard, Rumored. Some rooms are marked N/A because rarity simply does not apply to them.
What rarity is and is not
Rarity is a hint about how often a room shows up and how situational it is. It is not a score. A Commonplace room you can use every day may be worth more to a run than a Rumored room that fits no route you can build.
Using rarity while drafting
When two rooms fit the same door, rarity can break the tie: the rarer room might carry a unique effect worth routing around. But if the common room solves your immediate problem, take it and move on.
Filter the room database by rarity to see which rooms share a tier and whether the tier matches a job you need.