Passive fire stopping is the sealing openings to prevent the spread of fire (including smoke and heat) from passing through building compartments.
The spread of a fire is contained by fire resisting compartments, which are subdivide in the building both vertically and horizontally.
Buildings must ensure that any openings and gaps are fire stopped to restrict the spread of both fire and smoke.
Approved document B states that all fire-rated elements must be reinstated once penetrated by any of the above examples. The standard also states that these penetrations should be kept to a minimum, as small as possible and fire stopped. Furthermore, all joints that are between fire separating elements must be fire stopped.