To supersede the requirements of Sentence 9.25.3.3.(2), which would otherwise require that joints be lapped not less than 100 mm, on the basis that the air barrier is installed below the floor-on-ground.
This is to limit the probability of discontinuity in soil gas barriers, where they are weighed down by a concrete slab [which clamps the joints], which could lead to the infiltration of soil gas (particularly radon), which could lead to negative effects on the air quality of indoor spaces, which could lead to harm to persons.