The Annulus, or annular space, is the space between a penetrant and whatever surrounds it, such as the sides of an opening or a sleeve, as the case may be.
Sometimes an annulus can be "offset", meaning that the penetrant is not centred. If the firestop was tested only to a perfectly centred opening, an offset may disqualify it for use in the field.
In practical terms, an offset penetrant or annulus may very well present an added degree of difficulty for passing the fire test. A metallic sleeve as well as a metallic penetrant will conduct heat through the firestop. Insulation on the bottom of floor systems or in the middle of wall systems, as is the case with caulking or paint based (soft) systems, will encourage more heat to travel through the sleeve and the penetrant, which can put more stress on the seal on the unexposed side. In the case of a thin (below 4") firestop mortar, there is also more potential hazard from tight annular spacing because metal expands during the fire, thus squeezing the firestop - only to rapidly shrink when hit with the hose-stream test, which can let water through, thus failing the H rating in Canada or the entire rating in the US, where hose-stream testing is mandatory. Such a failure may no longer be visible once the test assembly has reached ambient temperatures again.
For all these considerations, the annular space is a critical aspect of field approvals.