The "Slice" function allows to divide a booking into individual bookings with different start and end dates. This functions is useful if for example a guest changes rooms during his stay.
When you use the Unit to select a different UNIT, if the symbol ® is next to a unit/room, this indicates there is already a booking in this unit, so you should choose another unit.
Which information you see on this tab is defined by the settings activated in SETTINGS->PROPERTIES->BOOKING QUESTIONS.