Factorio Forums

While this is obvious to any player doing quality seriously, it is in fact missing from in-game sources. The quality tip only says that quality is a property of an entity. However, hitpoint / spoilage are also properties of an entity but function differently: things with different hitpoint / spoilage still stack. Quality in fact makes a thing a different entity so it's not very accurate to describe it as a property.

Another problem is that none of the quality tips has a UI screenshot with quality buttons. This can be very misleading.