In addition to "core" properties: quantity, priority, owner and comment you can also create your own occurrence properties. This can be anything from numbers needed in the project to the color of a chair.
Unlike the other properties, the occurrence data is always unique to the specific item in a room and is not inherited from the template (the template cannot have occurrence data). This means that even though two rooms have the same item list from a room template they can have different values between the each individual occurrence in each room (e.g. color or number).
There are numerous use cases where this can be utilized, but there is always a trade of between choosing to storing data on the occurrence vs. storing it on the Item (type). You can use occurrence data to allow for small variances that should not affect the room templates or in cases where there is a property more related to "where" than "how". E.g. color should vary depending on floor level, but the quantity of each type should be the same. If you use occurrence data you cannot use templates to save you from updating each record.
When changing multiple occurrences you can also update custom occurrence data, but note that it would only allow you to change properties that are available on all the occurrences.
See admin guide for how to create new properties (dynamic GUI) and how to control on what Item the property is shown (Item view filter).