From the mailing list: When we receive physical books, most of them go to the barter club, the oldest ones, to the collector's club, but those of our subject are sent to our Library (this is specified in a field). These books require special fields, which we have in a specific tab. But in most cases, we don't need to see that tab, only when they belong to the library. The data entry dialog could have a general behavior, without these tabs of conditional visibility. But when a library record is selected, that tab would appear.